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<li><a class="reference internal" href="#">What’s New In Python 3.9</a><ul>
<li><a class="reference internal" href="#summary-release-highlights">Summary – Release highlights</a></li>
<li><a class="reference internal" href="#you-should-check-for-deprecationwarning-in-your-code">You should check for DeprecationWarning in your code</a></li>
<li><a class="reference internal" href="#new-features">New Features</a><ul>
<li><a class="reference internal" href="#dictionary-merge-update-operators">Dictionary Merge &amp; Update Operators</a></li>
<li><a class="reference internal" href="#new-string-methods-to-remove-prefixes-and-suffixes">New String Methods to Remove Prefixes and Suffixes</a></li>
<li><a class="reference internal" href="#type-hinting-generics-in-standard-collections">Type Hinting Generics in Standard Collections</a></li>
<li><a class="reference internal" href="#new-parser">New Parser</a></li>
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<li><a class="reference internal" href="#tracemalloc">tracemalloc</a></li>
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<li><a class="reference internal" href="#porting-to-python-3-9">Porting to Python 3.9</a><ul>
<li><a class="reference internal" href="#changes-in-the-python-api">Changes in the Python API</a></li>
<li><a class="reference internal" href="#changes-in-the-c-api">Changes in the C API</a></li>
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<li><a class="reference internal" href="#id2">Porting to Python 3.9</a></li>
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<li><a class="reference internal" href="#notable-changes-in-python-3-9-1">Notable changes in Python 3.9.1</a><ul>
<li><a class="reference internal" href="#id4">typing</a></li>
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<li><a class="reference internal" href="#notable-changes-in-python-3-9-2">Notable changes in Python 3.9.2</a><ul>
<li><a class="reference internal" href="#collections-abc">collections.abc</a></li>
<li><a class="reference internal" href="#urllib-parse">urllib.parse</a></li>
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 <section id="what-s-new-in-python-3-9">
<h1>What’s New In Python 3.9<a class="headerlink" href="#what-s-new-in-python-3-9" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Editor</dt>
<dd class="field-odd">Łukasz Langa
</dd>
</dl>
This article explains the new features in Python 3.9, compared to 3.8.
Python 3.9 was released on October 5, 2020.
For full details, see the <a class="reference internal" href="changelog.html#changelog">changelog</a>.
<div class="admonition seealso">
See also
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0596">PEP 596</a> - Python 3.9 Release Schedule
</div>
<section id="summary-release-highlights">
<h2>Summary – Release highlights<a class="headerlink" href="#summary-release-highlights" title="Permalink to this headline">¶</a></h2>
New syntax features:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0584">PEP 584</a>, union operators added to <code class="docutils literal notranslate">dict</code>;</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0585">PEP 585</a>, type hinting generics in standard collections;</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0614">PEP 614</a>, relaxed grammar restrictions on decorators.</li>
</ul>
New built-in features:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0616">PEP 616</a>, string methods to remove prefixes and suffixes.</li>
</ul>
New features in the standard library:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0593">PEP 593</a>, flexible function and variable annotations;</li>
<li><a class="reference internal" href="../library/os.html#os.pidfd_open" title="os.pidfd_open"><code class="xref py py-func docutils literal notranslate">os.pidfd_open()</code></a> added that allows process management without races
and signals.</li>
</ul>
Interpreter improvements:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0573">PEP 573</a>, fast access to module state from methods of C extension
types;</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0617">PEP 617</a>, CPython now uses a new parser based on PEG;</li>
<li>a number of Python builtins (range, tuple, set, frozenset, list, dict) are
now sped up using <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0590">PEP 590</a> vectorcall;</li>
<li>garbage collection does not block on resurrected objects;</li>
<li>a number of Python modules (<code class="xref py py-mod docutils literal notranslate">_abc</code>, <a class="reference internal" href="../library/audioop.html#module-audioop" title="audioop: Manipulate raw audio data. (deprecated)"><code class="xref py py-mod docutils literal notranslate">audioop</code></a>, <code class="xref py py-mod docutils literal notranslate">_bz2</code>,
<code class="xref py py-mod docutils literal notranslate">_codecs</code>, <code class="xref py py-mod docutils literal notranslate">_contextvars</code>, <code class="xref py py-mod docutils literal notranslate">_crypt</code>, <code class="xref py py-mod docutils literal notranslate">_functools</code>,
<code class="xref py py-mod docutils literal notranslate">_json</code>, <code class="xref py py-mod docutils literal notranslate">_locale</code>, <a class="reference internal" href="../library/math.html#module-math" title="math: Mathematical functions (sin() etc.)."><code class="xref py py-mod docutils literal notranslate">math</code></a>, <a class="reference internal" href="../library/operator.html#module-operator" title="operator: Functions corresponding to the standard operators."><code class="xref py py-mod docutils literal notranslate">operator</code></a>, <a class="reference internal" href="../library/resource.html#module-resource" title="resource: An interface to provide resource usage information on the current process. (Unix)"><code class="xref py py-mod docutils literal notranslate">resource</code></a>,
<a class="reference internal" href="../library/time.html#module-time" title="time: Time access and conversions."><code class="xref py py-mod docutils literal notranslate">time</code></a>, <code class="xref py py-mod docutils literal notranslate">_weakref</code>) now use multiphase initialization as defined
by PEP 489;</li>
<li>a number of standard library modules (<a class="reference internal" href="../library/audioop.html#module-audioop" title="audioop: Manipulate raw audio data. (deprecated)"><code class="xref py py-mod docutils literal notranslate">audioop</code></a>, <a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a>, <a class="reference internal" href="../library/grp.html#module-grp" title="grp: The group database (getgrnam() and friends). (Unix)"><code class="xref py py-mod docutils literal notranslate">grp</code></a>,
<code class="xref py py-mod docutils literal notranslate">_hashlib</code>, <a class="reference internal" href="../library/pwd.html#module-pwd" title="pwd: The password database (getpwnam() and friends). (Unix)"><code class="xref py py-mod docutils literal notranslate">pwd</code></a>, <code class="xref py py-mod docutils literal notranslate">_posixsubprocess</code>, <a class="reference internal" href="../library/random.html#module-random" title="random: Generate pseudo-random numbers with various common distributions."><code class="xref py py-mod docutils literal notranslate">random</code></a>,
<a class="reference internal" href="../library/select.html#module-select" title="select: Wait for I/O completion on multiple streams."><code class="xref py py-mod docutils literal notranslate">select</code></a>, <a class="reference internal" href="../library/struct.html#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate">struct</code></a>, <a class="reference internal" href="../library/termios.html#module-termios" title="termios: POSIX style tty control. (Unix)"><code class="xref py py-mod docutils literal notranslate">termios</code></a>, <a class="reference internal" href="../library/zlib.html#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><code class="xref py py-mod docutils literal notranslate">zlib</code></a>) are now using
the stable ABI defined by PEP 384.</li>
</ul>
New library modules:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0615">PEP 615</a>, the IANA Time Zone Database is now present in the standard
library in the <a class="reference internal" href="../library/zoneinfo.html#module-zoneinfo" title="zoneinfo: IANA time zone support"><code class="xref py py-mod docutils literal notranslate">zoneinfo</code></a> module;</li>
<li>an implementation of a topological sort of a graph is now provided in
the new <a class="reference internal" href="../library/graphlib.html#module-graphlib" title="graphlib: Functionality to operate with graph-like structures"><code class="xref py py-mod docutils literal notranslate">graphlib</code></a> module.</li>
</ul>
Release process changes:
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0602">PEP 602</a>, CPython adopts an annual release cycle.</li>
</ul>
</section>
<section id="you-should-check-for-deprecationwarning-in-your-code">
<h2>You should check for DeprecationWarning in your code<a class="headerlink" href="#you-should-check-for-deprecationwarning-in-your-code" title="Permalink to this headline">¶</a></h2>
When Python 2.7 was still supported, a lot of functionality in Python 3
was kept for backward compatibility with Python 2.7. With the end of Python
2 support, these backward compatibility layers have been removed, or will
be removed soon. Most of them emitted a <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> warning for
several years. For example, using <code class="docutils literal notranslate">collections.Mapping</code> instead of
<code class="docutils literal notranslate">collections.abc.Mapping</code> emits a <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> since Python
3.3, released in 2012.
Test your application with the <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><code class="xref std std-option docutils literal notranslate">-W</code></a> <code class="docutils literal notranslate">default</code> command-line option to see
<a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> and <a class="reference internal" href="../library/exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><code class="xref py py-exc docutils literal notranslate">PendingDeprecationWarning</code></a>, or even with
<a class="reference internal" href="../using/cmdline.html#cmdoption-W"><code class="xref std std-option docutils literal notranslate">-W</code></a> <code class="docutils literal notranslate">error</code> to treat them as errors. <a class="reference internal" href="../library/warnings.html#warning-filter">Warnings Filter</a> can be used to ignore warnings from third-party code.
Python 3.9 is the last version providing those Python 2 backward compatibility
layers, to give more time to Python projects maintainers to organize the
removal of the Python 2 support and add support for Python 3.9.
Aliases to <a class="reference internal" href="../library/collections.abc.html#collections-abstract-base-classes">Abstract Base Classes</a> in
the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate">collections</code></a> module, like <code class="docutils literal notranslate">collections.Mapping</code> alias to
<a class="reference internal" href="../library/collections.abc.html#collections.abc.Mapping" title="collections.abc.Mapping"><code class="xref py py-class docutils literal notranslate">collections.abc.Mapping</code></a>, are kept for one last release for backward
compatibility. They will be removed from Python 3.10.
More generally, try to run your tests in the <a class="reference internal" href="../library/devmode.html#devmode">Python Development Mode</a> which helps to prepare your code to make it compatible with the
next Python version.
Note: a number of pre-existing deprecations were removed in this version of
Python as well. Consult the <a class="reference internal" href="#removed-in-python-39">Removed</a> section.
</section>
<section id="new-features">
<h2>New Features<a class="headerlink" href="#new-features" title="Permalink to this headline">¶</a></h2>
<section id="dictionary-merge-update-operators">
<h3>Dictionary Merge &amp; Update Operators<a class="headerlink" href="#dictionary-merge-update-operators" title="Permalink to this headline">¶</a></h3>
Merge (<code class="docutils literal notranslate">|</code>) and update (<code class="docutils literal notranslate">|=</code>) operators have been added to the built-in
<a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> class. Those complement the existing <code class="docutils literal notranslate">dict.update</code> and
<code class="docutils literal notranslate">{**d1, **d2}</code> methods of merging dictionaries.
Example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = {&quot;key1&quot;: &quot;value1 from x&quot;, &quot;key2&quot;: &quot;value2 from x&quot;}
&gt;&gt;&gt; y = {&quot;key2&quot;: &quot;value2 from y&quot;, &quot;key3&quot;: &quot;value3 from y&quot;}
&gt;&gt;&gt; x | y
{&#39;key1&#39;: &#39;value1 from x&#39;, &#39;key2&#39;: &#39;value2 from y&#39;, &#39;key3&#39;: &#39;value3 from y&#39;}
&gt;&gt;&gt; y | x
{&#39;key2&#39;: &#39;value2 from x&#39;, &#39;key3&#39;: &#39;value3 from y&#39;, &#39;key1&#39;: &#39;value1 from x&#39;}
</pre></div>
</div>
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0584">PEP 584</a> for a full description.
(Contributed by Brandt Bucher in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36144">bpo-36144</a>.)
</section>
<section id="new-string-methods-to-remove-prefixes-and-suffixes">
<h3>New String Methods to Remove Prefixes and Suffixes<a class="headerlink" href="#new-string-methods-to-remove-prefixes-and-suffixes" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/stdtypes.html#str.removeprefix" title="str.removeprefix"><code class="xref py py-meth docutils literal notranslate">str.removeprefix(prefix)</code></a> and
<a class="reference internal" href="../library/stdtypes.html#str.removesuffix" title="str.removesuffix"><code class="xref py py-meth docutils literal notranslate">str.removesuffix(suffix)</code></a> have been added
to easily remove an unneeded prefix or a suffix from a string. Corresponding
<code class="docutils literal notranslate">bytes</code>, <code class="docutils literal notranslate">bytearray</code>, and <code class="docutils literal notranslate">collections.UserString</code> methods have also been
added. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0616">PEP 616</a> for a full description. (Contributed by Dennis Sweeney in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39939">bpo-39939</a>.)
</section>
<section id="type-hinting-generics-in-standard-collections">
<h3>Type Hinting Generics in Standard Collections<a class="headerlink" href="#type-hinting-generics-in-standard-collections" title="Permalink to this headline">¶</a></h3>
In type annotations you can now use built-in collection types such as
<code class="docutils literal notranslate">list</code> and <code class="docutils literal notranslate">dict</code> as generic types instead of importing the
corresponding capitalized types (e.g. <code class="docutils literal notranslate">List</code> or <code class="docutils literal notranslate">Dict</code>) from
<code class="docutils literal notranslate">typing</code>. Some other types in the standard library are also now generic,
for example <code class="docutils literal notranslate">queue.Queue</code>.
Example:
<div class="highlight-python notranslate"><div class="highlight"><pre>def greet_all(names: list[str]) -&gt; None:
 for name in names:
 print(&quot;Hello&quot;, name)
</pre></div>
</div>
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0585">PEP 585</a> for more details. (Contributed by Guido van Rossum,
Ethan Smith, and Batuhan Taşkaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39481">bpo-39481</a>.)
</section>
<section id="new-parser">
<h3>New Parser<a class="headerlink" href="#new-parser" title="Permalink to this headline">¶</a></h3>
Python 3.9 uses a new parser, based on <a class="reference external" href="https://en.wikipedia.org/wiki/Parsing_expression_grammar">PEG</a> instead
of <a class="reference external" href="https://en.wikipedia.org/wiki/LL_parser">LL(1)</a>. The new
parser’s performance is roughly comparable to that of the old parser,
but the PEG formalism is more flexible than LL(1) when it comes to
designing new language features. We’ll start using this flexibility
in Python 3.10 and later.
The <a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> module uses the new parser and produces the same AST as
the old parser.
In Python 3.10, the old parser will be deleted and so will all
functionality that depends on it (primarily the <code class="xref py py-mod docutils literal notranslate">parser</code> module,
which has long been deprecated). In Python 3.9 only, you can switch
back to the LL(1) parser using a command line switch (<code class="docutils literal notranslate">-X
oldparser</code>) or an environment variable (<code class="docutils literal notranslate">PYTHONOLDPARSER=1</code>).
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0617">PEP 617</a> for more details. (Contributed by Guido van Rossum,
Pablo Galindo and Lysandros Nikolaou in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40334">bpo-40334</a>.)
</section>
</section>
<section id="other-language-changes">
<h2>Other Language Changes<a class="headerlink" href="#other-language-changes" title="Permalink to this headline">¶</a></h2>
<ul>
<li><a class="reference internal" href="../library/functions.html#import__" title="__import__"><code class="xref py py-func docutils literal notranslate">__import__()</code></a> now raises <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a> instead of
<a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>, which used to occur when a relative import went past
its top-level package.
(Contributed by Ngalim Siregar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37444">bpo-37444</a>.)</li>
<li>Python now gets the absolute path of the script filename specified on
the command line (ex: <code class="docutils literal notranslate">python3 script.py</code>): the <code class="docutils literal notranslate">__file__</code> attribute of
the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``."><code class="xref py py-mod docutils literal notranslate">__main__</code></a> module became an absolute path, rather than a relative
path. These paths now remain valid after the current directory is changed
by <a class="reference internal" href="../library/os.html#os.chdir" title="os.chdir"><code class="xref py py-func docutils literal notranslate">os.chdir()</code></a>. As a side effect, the traceback also displays the
absolute path for <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``."><code class="xref py py-mod docutils literal notranslate">__main__</code></a> module frames in this case.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=20443">bpo-20443</a>.)</li>
<li>In the <a class="reference internal" href="../library/devmode.html#devmode">Python Development Mode</a> and in <a class="reference internal" href="../using/configure.html#debug-build">debug build</a>, the
encoding and errors arguments are now checked for string encoding and
decoding operations. Examples: <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a>, <a class="reference internal" href="../library/stdtypes.html#str.encode" title="str.encode"><code class="xref py py-meth docutils literal notranslate">str.encode()</code></a> and
<a class="reference internal" href="../library/stdtypes.html#bytes.decode" title="bytes.decode"><code class="xref py py-meth docutils literal notranslate">bytes.decode()</code></a>.
By default, for best performance, the errors argument is only checked at
the first encoding/decoding error and the encoding argument is sometimes
ignored for empty strings.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37388">bpo-37388</a>.)
</li>
<li><code class="docutils literal notranslate">&quot;&quot;.replace(&quot;&quot;, s, n)</code> now returns <code class="docutils literal notranslate">s</code> instead of an empty string for
all non-zero <code class="docutils literal notranslate">n</code>. It is now consistent with <code class="docutils literal notranslate">&quot;&quot;.replace(&quot;&quot;, s)</code>.
There are similar changes for <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> and <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a> objects.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=28029">bpo-28029</a>.)</li>
<li>Any valid expression can now be used as a <a class="reference internal" href="../glossary.html#term-decorator">decorator</a>. Previously, the
grammar was much more restrictive. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0614">PEP 614</a> for details.
(Contributed by Brandt Bucher in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39702">bpo-39702</a>.)</li>
<li>Improved help for the <a class="reference internal" href="../library/typing.html#module-typing" title="typing: Support for type hints (see :pep:`484`)."><code class="xref py py-mod docutils literal notranslate">typing</code></a> module. Docstrings are now shown for
all special forms and special generic aliases (like <code class="docutils literal notranslate">Union</code> and <code class="docutils literal notranslate">List</code>).
Using <a class="reference internal" href="../library/functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate">help()</code></a> with generic alias like <code class="docutils literal notranslate">List[int]</code> will show the help
for the correspondent concrete type (<code class="docutils literal notranslate">list</code> in this case).
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40257">bpo-40257</a>.)</li>
<li>Parallel running of <a class="reference internal" href="../reference/expressions.html#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate">aclose()</code></a> / <a class="reference internal" href="../reference/expressions.html#agen.asend" title="agen.asend"><code class="xref py py-meth docutils literal notranslate">asend()</code></a> /
<a class="reference internal" href="../reference/expressions.html#agen.athrow" title="agen.athrow"><code class="xref py py-meth docutils literal notranslate">athrow()</code></a> is now prohibited, and <code class="docutils literal notranslate">ag_running</code> now reflects
the actual running status of the async generator.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30773">bpo-30773</a>.)</li>
<li>Unexpected errors in calling the <code class="docutils literal notranslate">__iter__</code> method are no longer masked by
<code class="docutils literal notranslate">TypeError</code> in the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> operator and functions
<a class="reference internal" href="../library/operator.html#operator.contains" title="operator.contains"><code class="xref py py-func docutils literal notranslate">contains()</code></a>, <a class="reference internal" href="../library/operator.html#operator.indexOf" title="operator.indexOf"><code class="xref py py-func docutils literal notranslate">indexOf()</code></a> and
<a class="reference internal" href="../library/operator.html#operator.countOf" title="operator.countOf"><code class="xref py py-func docutils literal notranslate">countOf()</code></a> of the <a class="reference internal" href="../library/operator.html#module-operator" title="operator: Functions corresponding to the standard operators."><code class="xref py py-mod docutils literal notranslate">operator</code></a> module.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40824">bpo-40824</a>.)</li>
<li>Unparenthesized lambda expressions can no longer be the expression part in an
<code class="docutils literal notranslate">if</code> clause in comprehensions and generator expressions. See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=41848">bpo-41848</a>
and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=43755">bpo-43755</a> for details.</li>
</ul>
</section>
<section id="new-modules">
<h2>New Modules<a class="headerlink" href="#new-modules" title="Permalink to this headline">¶</a></h2>
<section id="zoneinfo">
<h3>zoneinfo<a class="headerlink" href="#zoneinfo" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/zoneinfo.html#module-zoneinfo" title="zoneinfo: IANA time zone support"><code class="xref py py-mod docutils literal notranslate">zoneinfo</code></a> module brings support for the IANA time zone database to
the standard library. It adds <a class="reference internal" href="../library/zoneinfo.html#zoneinfo.ZoneInfo" title="zoneinfo.ZoneInfo"><code class="xref py py-class docutils literal notranslate">zoneinfo.ZoneInfo</code></a>, a concrete
<a class="reference internal" href="../library/datetime.html#datetime.tzinfo" title="datetime.tzinfo"><code class="xref py py-class docutils literal notranslate">datetime.tzinfo</code></a> implementation backed by the system’s time zone data.
Example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from zoneinfo import ZoneInfo
&gt;&gt;&gt; from datetime import datetime, timedelta

&gt;&gt;&gt; # Daylight saving time
&gt;&gt;&gt; dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo(&quot;America/Los_Angeles&quot;))
&gt;&gt;&gt; print(dt)
2020-10-31 12:00:00-07:00
&gt;&gt;&gt; dt.tzname()
&#39;PDT&#39;

&gt;&gt;&gt; # Standard time
&gt;&gt;&gt; dt += timedelta(days=7)
&gt;&gt;&gt; print(dt)
2020-11-07 12:00:00-08:00
&gt;&gt;&gt; print(dt.tzname())
PST
</pre></div>
</div>
As a fall-back source of data for platforms that don’t ship the IANA database,
the <a class="reference external" href="https://pypi.org/project/tzdata/"><code class="docutils literal notranslate">tzdata</code></a> module was released as a first-party package – distributed via
PyPI and maintained by the CPython core team.
<div class="admonition seealso">
See also
<dl class="simple">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0615">PEP 615</a> – Support for the IANA Time Zone Database in the Standard Library</dt><dd>PEP written and implemented by Paul Ganssle
</dd>
</dl>
</div>
</section>
<section id="graphlib">
<h3>graphlib<a class="headerlink" href="#graphlib" title="Permalink to this headline">¶</a></h3>
A new module, <a class="reference internal" href="../library/graphlib.html#module-graphlib" title="graphlib: Functionality to operate with graph-like structures"><code class="xref py py-mod docutils literal notranslate">graphlib</code></a>, was added that contains the
<a class="reference internal" href="../library/graphlib.html#graphlib.TopologicalSorter" title="graphlib.TopologicalSorter"><code class="xref py py-class docutils literal notranslate">graphlib.TopologicalSorter</code></a> class to offer functionality to perform
topological sorting of graphs. (Contributed by Pablo Galindo, Tim Peters and
Larry Hastings in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17005">bpo-17005</a>.)
</section>
</section>
<section id="improved-modules">
<h2>Improved Modules<a class="headerlink" href="#improved-modules" title="Permalink to this headline">¶</a></h2>
<section id="ast">
<h3>ast<a class="headerlink" href="#ast" title="Permalink to this headline">¶</a></h3>
Added the indent option to <a class="reference internal" href="../library/ast.html#ast.dump" title="ast.dump"><code class="xref py py-func docutils literal notranslate">dump()</code></a> which allows it to produce a
multiline indented output.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37995">bpo-37995</a>.)
Added <a class="reference internal" href="../library/ast.html#ast.unparse" title="ast.unparse"><code class="xref py py-func docutils literal notranslate">ast.unparse()</code></a> as a function in the <a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> module that can
be used to unparse an <a class="reference internal" href="../library/ast.html#ast.AST" title="ast.AST"><code class="xref py py-class docutils literal notranslate">ast.AST</code></a> object and produce a string with code
that would produce an equivalent <a class="reference internal" href="../library/ast.html#ast.AST" title="ast.AST"><code class="xref py py-class docutils literal notranslate">ast.AST</code></a> object when parsed.
(Contributed by Pablo Galindo and Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38870">bpo-38870</a>.)
Added docstrings to AST nodes that contains the ASDL signature used to
construct that node. (Contributed by Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39638">bpo-39638</a>.)
</section>
<section id="asyncio">
<h3>asyncio<a class="headerlink" href="#asyncio" title="Permalink to this headline">¶</a></h3>
Due to significant security concerns, the reuse_address parameter of
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.create_datagram_endpoint" title="asyncio.loop.create_datagram_endpoint"><code class="xref py py-meth docutils literal notranslate">asyncio.loop.create_datagram_endpoint()</code></a> is no longer supported. This is
because of the behavior of the socket option <code class="docutils literal notranslate">SO_REUSEADDR</code> in UDP. For more
details, see the documentation for <code class="docutils literal notranslate">loop.create_datagram_endpoint()</code>.
(Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37228">bpo-37228</a>.)
Added a new <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a> <a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.shutdown_default_executor" title="asyncio.loop.shutdown_default_executor"><code class="xref py py-meth docutils literal notranslate">shutdown_default_executor()</code></a>
that schedules a shutdown for the default executor that waits on the
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">ThreadPoolExecutor</code></a> to finish closing. Also,
<a class="reference internal" href="../library/asyncio-task.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate">asyncio.run()</code></a> has been updated to use the new <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a>.
(Contributed by Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34037">bpo-34037</a>.)
Added <a class="reference internal" href="../library/asyncio-policy.html#asyncio.PidfdChildWatcher" title="asyncio.PidfdChildWatcher"><code class="xref py py-class docutils literal notranslate">asyncio.PidfdChildWatcher</code></a>, a Linux-specific child watcher
implementation that polls process file descriptors. (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38692">bpo-38692</a>)
Added a new <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a> <a class="reference internal" href="../library/asyncio-task.html#asyncio.to_thread" title="asyncio.to_thread"><code class="xref py py-func docutils literal notranslate">asyncio.to_thread()</code></a>. It is mainly used for
running IO-bound functions in a separate thread to avoid blocking the event
loop, and essentially works as a high-level version of
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate">run_in_executor()</code></a> that can directly take keyword arguments.
(Contributed by Kyle Stanley and Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32309">bpo-32309</a>.)
When cancelling the task due to a timeout, <a class="reference internal" href="../library/asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-meth docutils literal notranslate">asyncio.wait_for()</code></a> will now
wait until the cancellation is complete also in the case when timeout is
&lt;= 0, like it does with positive timeouts.
(Contributed by Elvis Pranskevichus in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32751">bpo-32751</a>.)
<a class="reference internal" href="../library/asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate">asyncio</code></a> now raises <code class="xref py py-exc docutils literal notranslate">TyperError</code> when calling incompatible
methods with an <a class="reference internal" href="../library/ssl.html#ssl.SSLSocket" title="ssl.SSLSocket"><code class="xref py py-class docutils literal notranslate">ssl.SSLSocket</code></a> socket.
(Contributed by Ido Michael in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37404">bpo-37404</a>.)
</section>
<section id="compileall">
<h3>compileall<a class="headerlink" href="#compileall" title="Permalink to this headline">¶</a></h3>
Added new possibility to use hardlinks for duplicated <code class="docutils literal notranslate">.pyc</code> files: hardlink_dupes parameter and –hardlink-dupes command line option.
(Contributed by Lumír ‘Frenzy’ Balhar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40495">bpo-40495</a>.)
Added new options for path manipulation in resulting <code class="docutils literal notranslate">.pyc</code> files: stripdir, prependdir, limit_sl_dest parameters and -s, -p, -e command line options.
Added the possibility to specify the option for an optimization level multiple times.
(Contributed by Lumír ‘Frenzy’ Balhar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38112">bpo-38112</a>.)
</section>
<section id="concurrent-futures">
<h3>concurrent.futures<a class="headerlink" href="#concurrent-futures" title="Permalink to this headline">¶</a></h3>
Added a new cancel_futures parameter to
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.Executor.shutdown" title="concurrent.futures.Executor.shutdown"><code class="xref py py-meth docutils literal notranslate">concurrent.futures.Executor.shutdown()</code></a> that cancels all pending futures
which have not started running, instead of waiting for them to complete before
shutting down the executor.
(Contributed by Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39349">bpo-39349</a>.)
Removed daemon threads from <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">ThreadPoolExecutor</code></a>
and <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate">ProcessPoolExecutor</code></a>. This improves
compatibility with subinterpreters and predictability in their shutdown
processes. (Contributed by Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39812">bpo-39812</a>.)
Workers in <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate">ProcessPoolExecutor</code></a> are now spawned on
demand, only when there are no available idle workers to reuse. This optimizes
startup overhead and reduces the amount of lost CPU time to idle workers.
(Contributed by Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39207">bpo-39207</a>.)
</section>
<section id="curses">
<h3>curses<a class="headerlink" href="#curses" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/curses.html#curses.get_escdelay" title="curses.get_escdelay"><code class="xref py py-func docutils literal notranslate">curses.get_escdelay()</code></a>, <a class="reference internal" href="../library/curses.html#curses.set_escdelay" title="curses.set_escdelay"><code class="xref py py-func docutils literal notranslate">curses.set_escdelay()</code></a>,
<a class="reference internal" href="../library/curses.html#curses.get_tabsize" title="curses.get_tabsize"><code class="xref py py-func docutils literal notranslate">curses.get_tabsize()</code></a>, and <a class="reference internal" href="../library/curses.html#curses.set_tabsize" title="curses.set_tabsize"><code class="xref py py-func docutils literal notranslate">curses.set_tabsize()</code></a> functions.
(Contributed by Anthony Sottile in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38312">bpo-38312</a>.)
</section>
<section id="datetime">
<h3>datetime<a class="headerlink" href="#datetime" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/datetime.html#datetime.date.isocalendar" title="datetime.date.isocalendar"><code class="xref py py-meth docutils literal notranslate">isocalendar()</code></a> of <a class="reference internal" href="../library/datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate">datetime.date</code></a>
and <a class="reference internal" href="../library/datetime.html#datetime.datetime.isocalendar" title="datetime.datetime.isocalendar"><code class="xref py py-meth docutils literal notranslate">isocalendar()</code></a> of <a class="reference internal" href="../library/datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate">datetime.datetime</code></a>
methods now returns a <a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">namedtuple()</code></a> instead of a <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a>.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=24416">bpo-24416</a>.)
</section>
<section id="distutils">
<h3>distutils<a class="headerlink" href="#distutils" title="Permalink to this headline">¶</a></h3>
The upload command now creates SHA2-256 and Blake2b-256 hash
digests. It skips MD5 on platforms that block MD5 digest.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40698">bpo-40698</a>.)
</section>
<section id="fcntl">
<h3>fcntl<a class="headerlink" href="#fcntl" title="Permalink to this headline">¶</a></h3>
Added constants <code class="xref py py-data docutils literal notranslate">F_OFD_GETLK</code>, <code class="xref py py-data docutils literal notranslate">F_OFD_SETLK</code>
and <code class="xref py py-data docutils literal notranslate">F_OFD_SETLKW</code>.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38602">bpo-38602</a>.)
</section>
<section id="ftplib">
<h3>ftplib<a class="headerlink" href="#ftplib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/ftplib.html#ftplib.FTP" title="ftplib.FTP"><code class="xref py py-class docutils literal notranslate">FTP</code></a> and <a class="reference internal" href="../library/ftplib.html#ftplib.FTP_TLS" title="ftplib.FTP_TLS"><code class="xref py py-class docutils literal notranslate">FTP_TLS</code></a> now raise a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-class docutils literal notranslate">ValueError</code></a>
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39259">bpo-39259</a>.)
</section>
<section id="gc">
<h3>gc<a class="headerlink" href="#gc" title="Permalink to this headline">¶</a></h3>
When the garbage collector makes a collection in which some objects resurrect
(they are reachable from outside the isolated cycles after the finalizers have
been executed), do not block the collection of all objects that are still
unreachable. (Contributed by Pablo Galindo and Tim Peters in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38379">bpo-38379</a>.)
Added a new function <a class="reference internal" href="../library/gc.html#gc.is_finalized" title="gc.is_finalized"><code class="xref py py-func docutils literal notranslate">gc.is_finalized()</code></a> to check if an object has been
finalized by the garbage collector. (Contributed by Pablo Galindo in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39322">bpo-39322</a>.)
</section>
<section id="hashlib">
<h3>hashlib<a class="headerlink" href="#hashlib" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate">hashlib</code></a> module can now use SHA3 hashes and SHAKE XOF from OpenSSL
when available.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37630">bpo-37630</a>.)
Builtin hash modules can now be disabled with
<code class="docutils literal notranslate">./configure --without-builtin-hashlib-hashes</code> or selectively enabled with
e.g. <code class="docutils literal notranslate">./configure --with-builtin-hashlib-hashes=sha3,blake2</code> to force use
of OpenSSL based implementation.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40479">bpo-40479</a>)
</section>
<section id="http">
<h3>http<a class="headerlink" href="#http" title="Permalink to this headline">¶</a></h3>
HTTP status codes <code class="docutils literal notranslate">103 EARLY_HINTS</code>, <code class="docutils literal notranslate">418 IM_A_TEAPOT</code> and <code class="docutils literal notranslate">425 TOO_EARLY</code> are added to
<a class="reference internal" href="../library/http.html#http.HTTPStatus" title="http.HTTPStatus"><code class="xref py py-class docutils literal notranslate">http.HTTPStatus</code></a>. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39509">bpo-39509</a> and Ross Rhodes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39507">bpo-39507</a>.)
</section>
<section id="idle-and-idlelib">
<h3>IDLE and idlelib<a class="headerlink" href="#idle-and-idlelib" title="Permalink to this headline">¶</a></h3>
Added option to toggle cursor blink off. (Contributed by Zackery Spytz
in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4603">bpo-4603</a>.)
Escape key now closes IDLE completion windows. (Contributed by Johnny
Najera in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38944">bpo-38944</a>.)
Added keywords to module name completion list. (Contributed by Terry J.
Reedy in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37765">bpo-37765</a>.)
New in 3.9 maintenance releases
Make IDLE invoke <a class="reference internal" href="../library/sys.html#sys.excepthook" title="sys.excepthook"><code class="xref py py-func docutils literal notranslate">sys.excepthook()</code></a> (when started without ‘-n’).
User hooks were previously ignored. (Contributed by Ken Hilton in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=43008">bpo-43008</a>.)
The changes above have been backported to 3.8 maintenance releases.
Rearrange the settings dialog. Split the General tab into Windows
and Shell/Ed tabs. Move help sources, which extend the Help menu, to the
Extensions tab. Make space for new options and shorten the dialog. The
latter makes the dialog better fit small screens. (Contributed by Terry Jan
Reedy in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40468">bpo-40468</a>.) Move the indent space setting from the Font tab to
the new Windows tab. (Contributed by Mark Roseman and Terry Jan Reedy in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33962">bpo-33962</a>.)
Apply syntax highlighting to <code class="docutils literal notranslate">.pyi</code> files. (Contributed by Alex
Waygood and Terry Jan Reedy in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=45447">bpo-45447</a>.)
</section>
<section id="imaplib">
<h3>imaplib<a class="headerlink" href="#imaplib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4" title="imaplib.IMAP4"><code class="xref py py-class docutils literal notranslate">IMAP4</code></a> and <a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4_SSL" title="imaplib.IMAP4_SSL"><code class="xref py py-class docutils literal notranslate">IMAP4_SSL</code></a> now have
an optional timeout parameter for their constructors.
Also, the <a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4.open" title="imaplib.IMAP4.open"><code class="xref py py-meth docutils literal notranslate">open()</code></a> method now has an optional timeout parameter
with this change. The overridden methods of <a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4_SSL" title="imaplib.IMAP4_SSL"><code class="xref py py-class docutils literal notranslate">IMAP4_SSL</code></a> and
<a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4_stream" title="imaplib.IMAP4_stream"><code class="xref py py-class docutils literal notranslate">IMAP4_stream</code></a> were applied to this change.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38615">bpo-38615</a>.)
<a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4.unselect" title="imaplib.IMAP4.unselect"><code class="xref py py-meth docutils literal notranslate">imaplib.IMAP4.unselect()</code></a> is added.
<a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4.unselect" title="imaplib.IMAP4.unselect"><code class="xref py py-meth docutils literal notranslate">imaplib.IMAP4.unselect()</code></a> frees server’s resources associated with the
selected mailbox and returns the server to the authenticated
state. This command performs the same actions as <a class="reference internal" href="../library/imaplib.html#imaplib.IMAP4.close" title="imaplib.IMAP4.close"><code class="xref py py-meth docutils literal notranslate">imaplib.IMAP4.close()</code></a>, except
that no messages are permanently removed from the currently
selected mailbox. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40375">bpo-40375</a>.)
</section>
<section id="importlib">
<h3>importlib<a class="headerlink" href="#importlib" title="Permalink to this headline">¶</a></h3>
To improve consistency with import statements, <a class="reference internal" href="../library/importlib.html#importlib.util.resolve_name" title="importlib.util.resolve_name"><code class="xref py py-func docutils literal notranslate">importlib.util.resolve_name()</code></a>
now raises <a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a> instead of <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> for invalid relative
import attempts.
(Contributed by Ngalim Siregar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37444">bpo-37444</a>.)
Import loaders which publish immutable module objects can now publish
immutable packages in addition to individual modules.
(Contributed by Dino Viehland in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39336">bpo-39336</a>.)
Added <a class="reference internal" href="../library/importlib.html#importlib.resources.files" title="importlib.resources.files"><code class="xref py py-func docutils literal notranslate">importlib.resources.files()</code></a> function with support for
subdirectories in package data, matching backport in <code class="docutils literal notranslate">importlib_resources</code>
version 1.5.
(Contributed by Jason R. Coombs in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39791">bpo-39791</a>.)
Refreshed <code class="docutils literal notranslate">importlib.metadata</code> from <code class="docutils literal notranslate">importlib_metadata</code> version 1.6.1.
</section>
<section id="inspect">
<h3>inspect<a class="headerlink" href="#inspect" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/inspect.html#inspect.BoundArguments.arguments" title="inspect.BoundArguments.arguments"><code class="xref py py-attr docutils literal notranslate">inspect.BoundArguments.arguments</code></a> is changed from <code class="docutils literal notranslate">OrderedDict</code> to regular
dict. (Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36350">bpo-36350</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39775">bpo-39775</a>.)
</section>
<section id="ipaddress">
<h3>ipaddress<a class="headerlink" href="#ipaddress" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/ipaddress.html#module-ipaddress" title="ipaddress: IPv4/IPv6 manipulation library."><code class="xref py py-mod docutils literal notranslate">ipaddress</code></a> now supports IPv6 Scoped Addresses (IPv6 address with suffix <code class="docutils literal notranslate">%&lt;scope_id&gt;</code>).
Scoped IPv6 addresses can be parsed using <a class="reference internal" href="../library/ipaddress.html#ipaddress.IPv6Address" title="ipaddress.IPv6Address"><code class="xref py py-class docutils literal notranslate">ipaddress.IPv6Address</code></a>.
If present, scope zone ID is available through the <a class="reference internal" href="../library/ipaddress.html#ipaddress.IPv6Address.scope_id" title="ipaddress.IPv6Address.scope_id"><code class="xref py py-attr docutils literal notranslate">scope_id</code></a> attribute.
(Contributed by Oleksandr Pavliuk in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34788">bpo-34788</a>.)
Starting with Python 3.9.5 the <a class="reference internal" href="../library/ipaddress.html#module-ipaddress" title="ipaddress: IPv4/IPv6 manipulation library."><code class="xref py py-mod docutils literal notranslate">ipaddress</code></a> module no longer
accepts any leading zeros in IPv4 address strings.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36384">bpo-36384</a>).
</section>
<section id="math">
<h3>math<a class="headerlink" href="#math" title="Permalink to this headline">¶</a></h3>
Expanded the <a class="reference internal" href="../library/math.html#math.gcd" title="math.gcd"><code class="xref py py-func docutils literal notranslate">math.gcd()</code></a> function to handle multiple arguments.
Formerly, it only supported two arguments.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39648">bpo-39648</a>.)
Added <a class="reference internal" href="../library/math.html#math.lcm" title="math.lcm"><code class="xref py py-func docutils literal notranslate">math.lcm()</code></a>: return the least common multiple of specified arguments.
(Contributed by Mark Dickinson, Ananthakrishnan and Serhiy Storchaka in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39479">bpo-39479</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39648">bpo-39648</a>.)
Added <a class="reference internal" href="../library/math.html#math.nextafter" title="math.nextafter"><code class="xref py py-func docutils literal notranslate">math.nextafter()</code></a>: return the next floating-point value after x
towards y.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39288">bpo-39288</a>.)
Added <a class="reference internal" href="../library/math.html#math.ulp" title="math.ulp"><code class="xref py py-func docutils literal notranslate">math.ulp()</code></a>: return the value of the least significant bit
of a float.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39310">bpo-39310</a>.)
</section>
<section id="multiprocessing">
<h3>multiprocessing<a class="headerlink" href="#multiprocessing" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/multiprocessing.html#multiprocessing.SimpleQueue" title="multiprocessing.SimpleQueue"><code class="xref py py-class docutils literal notranslate">multiprocessing.SimpleQueue</code></a> class has a new
<a class="reference internal" href="../library/multiprocessing.html#multiprocessing.SimpleQueue.close" title="multiprocessing.SimpleQueue.close"><code class="xref py py-meth docutils literal notranslate">close()</code></a> method to explicitly close the
queue.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30966">bpo-30966</a>.)
</section>
<section id="nntplib">
<h3>nntplib<a class="headerlink" href="#nntplib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/nntplib.html#nntplib.NNTP" title="nntplib.NNTP"><code class="xref py py-class docutils literal notranslate">NNTP</code></a> and <a class="reference internal" href="../library/nntplib.html#nntplib.NNTP_SSL" title="nntplib.NNTP_SSL"><code class="xref py py-class docutils literal notranslate">NNTP_SSL</code></a> now raise a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-class docutils literal notranslate">ValueError</code></a>
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39259">bpo-39259</a>.)
</section>
<section id="os">
<h3>os<a class="headerlink" href="#os" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/os.html#os.CLD_KILLED" title="os.CLD_KILLED"><code class="xref py py-data docutils literal notranslate">CLD_KILLED</code></a> and <a class="reference internal" href="../library/os.html#os.CLD_STOPPED" title="os.CLD_STOPPED"><code class="xref py py-data docutils literal notranslate">CLD_STOPPED</code></a> for <code class="xref py py-attr docutils literal notranslate">si_code</code>.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38493">bpo-38493</a>.)
Exposed the Linux-specific <a class="reference internal" href="../library/os.html#os.pidfd_open" title="os.pidfd_open"><code class="xref py py-func docutils literal notranslate">os.pidfd_open()</code></a> (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38692">bpo-38692</a>) and
<a class="reference internal" href="../library/os.html#os.P_PIDFD" title="os.P_PIDFD"><code class="xref py py-data docutils literal notranslate">os.P_PIDFD</code></a> (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38713">bpo-38713</a>) for process management with file
descriptors.
The <a class="reference internal" href="../library/os.html#os.unsetenv" title="os.unsetenv"><code class="xref py py-func docutils literal notranslate">os.unsetenv()</code></a> function is now also available on Windows.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39413">bpo-39413</a>.)
The <a class="reference internal" href="../library/os.html#os.putenv" title="os.putenv"><code class="xref py py-func docutils literal notranslate">os.putenv()</code></a> and <a class="reference internal" href="../library/os.html#os.unsetenv" title="os.unsetenv"><code class="xref py py-func docutils literal notranslate">os.unsetenv()</code></a> functions are now always
available.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39395">bpo-39395</a>.)
Added <a class="reference internal" href="../library/os.html#os.waitstatus_to_exitcode" title="os.waitstatus_to_exitcode"><code class="xref py py-func docutils literal notranslate">os.waitstatus_to_exitcode()</code></a> function:
convert a wait status to an exit code.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40094">bpo-40094</a>.)
</section>
<section id="pathlib">
<h3>pathlib<a class="headerlink" href="#pathlib" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/pathlib.html#pathlib.Path.readlink" title="pathlib.Path.readlink"><code class="xref py py-meth docutils literal notranslate">pathlib.Path.readlink()</code></a> which acts similarly to
<a class="reference internal" href="../library/os.html#os.readlink" title="os.readlink"><code class="xref py py-func docutils literal notranslate">os.readlink()</code></a>.
(Contributed by Girts Folkmanis in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30618">bpo-30618</a>)
</section>
<section id="pdb">
<h3>pdb<a class="headerlink" href="#pdb" title="Permalink to this headline">¶</a></h3>
On Windows now <a class="reference internal" href="../library/pdb.html#pdb.Pdb" title="pdb.Pdb"><code class="xref py py-class docutils literal notranslate">Pdb</code></a> supports <code class="docutils literal notranslate">~/.pdbrc</code>.
(Contributed by Tim Hopper and Dan Lidral-Porter in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=20523">bpo-20523</a>.)
</section>
<section id="poplib">
<h3>poplib<a class="headerlink" href="#poplib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/poplib.html#poplib.POP3" title="poplib.POP3"><code class="xref py py-class docutils literal notranslate">POP3</code></a> and <a class="reference internal" href="../library/poplib.html#poplib.POP3_SSL" title="poplib.POP3_SSL"><code class="xref py py-class docutils literal notranslate">POP3_SSL</code></a> now raise a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-class docutils literal notranslate">ValueError</code></a>
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39259">bpo-39259</a>.)
</section>
<section id="pprint">
<h3>pprint<a class="headerlink" href="#pprint" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/pprint.html#module-pprint" title="pprint: Data pretty printer."><code class="xref py py-mod docutils literal notranslate">pprint</code></a> can now pretty-print <a class="reference internal" href="../library/types.html#types.SimpleNamespace" title="types.SimpleNamespace"><code class="xref py py-class docutils literal notranslate">types.SimpleNamespace</code></a>.
(Contributed by Carl Bordum Hansen in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37376">bpo-37376</a>.)
</section>
<section id="pydoc">
<h3>pydoc<a class="headerlink" href="#pydoc" title="Permalink to this headline">¶</a></h3>
The documentation string is now shown not only for class, function,
method etc, but for any object that has its own <code class="docutils literal notranslate">__doc__</code> attribute.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40257">bpo-40257</a>.)
</section>
<section id="random">
<h3>random<a class="headerlink" href="#random" title="Permalink to this headline">¶</a></h3>
Added a new <code class="xref py py-attr docutils literal notranslate">random.Random.randbytes</code> method: generate random bytes.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40286">bpo-40286</a>.)
</section>
<section id="signal">
<h3>signal<a class="headerlink" href="#signal" title="Permalink to this headline">¶</a></h3>
Exposed the Linux-specific <a class="reference internal" href="../library/signal.html#signal.pidfd_send_signal" title="signal.pidfd_send_signal"><code class="xref py py-func docutils literal notranslate">signal.pidfd_send_signal()</code></a> for sending to
signals to a process using a file descriptor instead of a pid. (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38712">bpo-38712</a>)
</section>
<section id="smtplib">
<h3>smtplib<a class="headerlink" href="#smtplib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/smtplib.html#smtplib.SMTP" title="smtplib.SMTP"><code class="xref py py-class docutils literal notranslate">SMTP</code></a> and <a class="reference internal" href="../library/smtplib.html#smtplib.SMTP_SSL" title="smtplib.SMTP_SSL"><code class="xref py py-class docutils literal notranslate">SMTP_SSL</code></a> now raise a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-class docutils literal notranslate">ValueError</code></a>
if the given timeout for their constructor is zero to prevent the creation of
a non-blocking socket. (Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39259">bpo-39259</a>.)
<a class="reference internal" href="../library/smtplib.html#smtplib.LMTP" title="smtplib.LMTP"><code class="xref py py-class docutils literal notranslate">LMTP</code></a> constructor now has an optional timeout parameter.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39329">bpo-39329</a>.)
</section>
<section id="socket">
<h3>socket<a class="headerlink" href="#socket" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/socket.html#module-socket" title="socket: Low-level networking interface."><code class="xref py py-mod docutils literal notranslate">socket</code></a> module now exports the <a class="reference internal" href="../library/socket.html#socket.CAN_RAW_JOIN_FILTERS" title="socket.CAN_RAW_JOIN_FILTERS"><code class="xref py py-data docutils literal notranslate">CAN_RAW_JOIN_FILTERS</code></a>
constant on Linux 4.1 and greater.
(Contributed by Stefan Tatschner and Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=25780">bpo-25780</a>.)
The socket module now supports the <a class="reference internal" href="../library/socket.html#socket.CAN_J1939" title="socket.CAN_J1939"><code class="xref py py-data docutils literal notranslate">CAN_J1939</code></a> protocol on
platforms that support it. (Contributed by Karl Ding in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40291">bpo-40291</a>.)
The socket module now has the <a class="reference internal" href="../library/socket.html#socket.send_fds" title="socket.send_fds"><code class="xref py py-func docutils literal notranslate">socket.send_fds()</code></a> and
<a class="reference internal" href="../library/socket.html#socket.recv_fds" title="socket.recv_fds"><code class="xref py py-func docutils literal notranslate">socket.recv_fds()</code></a> functions. (Contributed by Joannah Nanjekye, Shinya
Okano and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=28724">bpo-28724</a>.)
</section>
<section id="time">
<h3>time<a class="headerlink" href="#time" title="Permalink to this headline">¶</a></h3>
On AIX, <a class="reference internal" href="../library/time.html#time.thread_time" title="time.thread_time"><code class="xref py py-func docutils literal notranslate">thread_time()</code></a> is now implemented with <code class="docutils literal notranslate">thread_cputime()</code>
which has nanosecond resolution, rather than
<code class="docutils literal notranslate">clock_gettime(CLOCK_THREAD_CPUTIME_ID)</code> which has a resolution of 10 ms.
(Contributed by Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40192">bpo-40192</a>)
</section>
<section id="sys">
<h3>sys<a class="headerlink" href="#sys" title="Permalink to this headline">¶</a></h3>
Added a new <a class="reference internal" href="../library/sys.html#sys.platlibdir" title="sys.platlibdir"><code class="xref py py-attr docutils literal notranslate">sys.platlibdir</code></a> attribute: name of the platform-specific
library directory. It is used to build the path of standard library and the
paths of installed extension modules. It is equal to <code class="docutils literal notranslate">&quot;lib&quot;</code> on most
platforms. On Fedora and SuSE, it is equal to <code class="docutils literal notranslate">&quot;lib64&quot;</code> on 64-bit platforms.
(Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1294959">bpo-1294959</a>.)
Previously, <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-attr docutils literal notranslate">sys.stderr</code></a> was block-buffered when non-interactive. Now
<code class="docutils literal notranslate">stderr</code> defaults to always being line-buffered.
(Contributed by Jendrik Seipp in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=13601">bpo-13601</a>.)
</section>
<section id="tracemalloc">
<h3>tracemalloc<a class="headerlink" href="#tracemalloc" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/tracemalloc.html#tracemalloc.reset_peak" title="tracemalloc.reset_peak"><code class="xref py py-func docutils literal notranslate">tracemalloc.reset_peak()</code></a> to set the peak size of traced memory
blocks to the current size, to measure the peak of specific pieces of code.
(Contributed by Huon Wilson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40630">bpo-40630</a>.)
</section>
<section id="typing">
<h3>typing<a class="headerlink" href="#typing" title="Permalink to this headline">¶</a></h3>
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0593">PEP 593</a> introduced an <a class="reference internal" href="../library/typing.html#typing.Annotated" title="typing.Annotated"><code class="xref py py-data docutils literal notranslate">typing.Annotated</code></a> type to decorate existing
types with context-specific metadata and new <code class="docutils literal notranslate">include_extras</code> parameter to
<a class="reference internal" href="../library/typing.html#typing.get_type_hints" title="typing.get_type_hints"><code class="xref py py-func docutils literal notranslate">typing.get_type_hints()</code></a> to access the metadata at runtime. (Contributed
by Till Varoquaux and Konstantin Kashin.)
</section>
<section id="unicodedata">
<h3>unicodedata<a class="headerlink" href="#unicodedata" title="Permalink to this headline">¶</a></h3>
The Unicode database has been updated to version 13.0.0. (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39926">bpo-39926</a>).
</section>
<section id="venv">
<h3>venv<a class="headerlink" href="#venv" title="Permalink to this headline">¶</a></h3>
The activation scripts provided by <a class="reference internal" href="../library/venv.html#module-venv" title="venv: Creation of virtual environments."><code class="xref py py-mod docutils literal notranslate">venv</code></a> now all specify their prompt
customization consistently by always using the value specified by
<code class="docutils literal notranslate">__VENV_PROMPT__</code>. Previously some scripts unconditionally used
<code class="docutils literal notranslate">__VENV_PROMPT__</code>, others only if it happened to be set (which was the default
case), and one used <code class="docutils literal notranslate">__VENV_NAME__</code> instead.
(Contributed by Brett Cannon in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37663">bpo-37663</a>.)
</section>
<section id="xml">
<h3>xml<a class="headerlink" href="#xml" title="Permalink to this headline">¶</a></h3>
White space characters within attributes are now preserved when serializing
<a class="reference internal" href="../library/xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal notranslate">xml.etree.ElementTree</code></a> to XML file. EOLNs are no longer normalized
to “n”. This is the result of discussion about how to interpret
section 2.11 of XML spec.
(Contributed by Mefistotelis in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39011">bpo-39011</a>.)
</section>
</section>
<section id="optimizations">
<h2>Optimizations<a class="headerlink" href="#optimizations" title="Permalink to this headline">¶</a></h2>
<ul>
<li>Optimized the idiom for assignment a temporary variable in comprehensions.
Now <code class="docutils literal notranslate">for y in [expr]</code> in comprehensions is as fast as a simple assignment
<code class="docutils literal notranslate">y = expr</code>. For example:
<blockquote>
<div>sums = [s for s in [0] for x in data for s in [s + x]]
</div></blockquote>
Unlike the <code class="docutils literal notranslate">:=</code> operator this idiom does not leak a variable to the
outer scope.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32856">bpo-32856</a>.)
</li>
<li>Optimized signal handling in multithreaded applications. If a thread different
than the main thread gets a signal, the bytecode evaluation loop is no longer
interrupted at each bytecode instruction to check for pending signals which
cannot be handled. Only the main thread of the main interpreter can handle
signals.
Previously, the bytecode evaluation loop was interrupted at each instruction
until the main thread handles signals.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40010">bpo-40010</a>.)
</li>
<li>Optimized the <a class="reference internal" href="../library/subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate">subprocess</code></a> module on FreeBSD using <code class="docutils literal notranslate">closefrom()</code>.
(Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor
Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38061">bpo-38061</a>.)</li>
<li><a class="reference internal" href="../c-api/long.html#c.PyLong_FromDouble" title="PyLong_FromDouble"><code class="xref c c-func docutils literal notranslate">PyLong_FromDouble()</code></a> is now up to 1.87x faster for values that
fit into long.
(Contributed by Sergey Fedoseev in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37986">bpo-37986</a>.)</li>
<li>A number of Python builtins (<a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-class docutils literal notranslate">range</code></a>, <a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a>, <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a>,
<a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate">frozenset</code></a>, <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a>, <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>) are now sped up by using
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0590">PEP 590</a> vectorcall protocol.
(Contributed by Dong-hee Na, Mark Shannon, Jeroen Demeyer and Petr Viktorin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37207">bpo-37207</a>.)</li>
<li>Optimized <code class="xref py py-func docutils literal notranslate">difference_update()</code> for the case when the other set
is much larger than the base set.
(Suggested by Evgeny Kapun with code contributed by Michele Orrù in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=8425">bpo-8425</a>.)</li>
<li>Python’s small object allocator (<code class="docutils literal notranslate">obmalloc.c</code>) now allows (no more than)
one empty arena to remain available for immediate reuse, without returning
it to the OS. This prevents thrashing in simple loops where an arena could
be created and destroyed anew on each iteration.
(Contributed by Tim Peters in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37257">bpo-37257</a>.)</li>
<li><a class="reference internal" href="../glossary.html#term-floor-division">floor division</a> of float operation now has a better performance. Also
the message of <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate">ZeroDivisionError</code></a> for this operation is updated.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39434">bpo-39434</a>.)</li>
<li>Decoding short ASCII strings with UTF-8 and ascii codecs is now about
15% faster. (Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37348">bpo-37348</a>.)</li>
</ul>
Here’s a summary of performance improvements from Python 3.4 through Python 3.9:
<div class="highlight-none notranslate"><div class="highlight"><pre>Python version 3.4 3.5 3.6 3.7 3.8 3.9
-------------- --- --- --- --- --- ---

Variable and attribute read access:
    read_local                       7.1     7.1     5.4     5.1     3.9    3.9
    read_nonlocal                    7.1     8.1     5.8     5.4     4.4    4.5
    read_global                     15.5    19.0    14.3    13.6     7.6    7.8
    read_builtin                    21.1    21.6    18.5    19.0     7.5    7.8
    read_classvar_from_class        25.6    26.5    20.7    19.5    18.4   17.9
    read_classvar_from_instance     22.8    23.5    18.8    17.1    16.4   16.9
    read_instancevar                32.4    33.1    28.0    26.3    25.4   25.3
    read_instancevar_slots          27.8    31.3    20.8    20.8    20.2   20.5
    read_namedtuple                 73.8    57.5    45.0    46.8    18.4   18.7
    read_boundmethod                37.6    37.9    29.6    26.9    27.7   41.1

Variable and attribute write access:
    write_local                      8.7     9.3     5.5     5.3     4.3    4.3
    write_nonlocal                  10.5    11.1     5.6     5.5     4.7    4.8
    write_global                    19.7    21.2    18.0    18.0    15.8   16.7
    write_classvar                  92.9    96.0   104.6   102.1    39.2   39.8
    write_instancevar               44.6    45.8    40.0    38.9    35.5   37.4
    write_instancevar_slots         35.6    36.1    27.3    26.6    25.7   25.8

Data structure read access:
    read_list                       24.2    24.5    20.8    20.8    19.0   19.5
    read_deque                      24.7    25.5    20.2    20.6    19.8   20.2
    read_dict                       24.3    25.7    22.3    23.0    21.0   22.4
    read_strdict                    22.6    24.3    19.5    21.2    18.9   21.5

Data structure write access:
    write_list                      27.1    28.5    22.5    21.6    20.0   20.0
    write_deque                     28.7    30.1    22.7    21.8    23.5   21.7
    write_dict                      31.4    33.3    29.3    29.2    24.7   25.4
    write_strdict                   28.4    29.9    27.5    25.2    23.1   24.5

Stack (or queue) operations:
    list_append_pop                 93.4   112.7    75.4    74.2    50.8   50.6
    deque_append_pop                43.5    57.0    49.4    49.2    42.5   44.2
    deque_append_popleft            43.7    57.3    49.7    49.7    42.8   46.4

Timing loop:
 loop_overhead 0.5 0.6 0.4 0.3 0.3 0.3
</pre></div>
</div>
These results were generated from the variable access benchmark script at:
<code class="docutils literal notranslate">Tools/scripts/var_access_benchmark.py</code>. The benchmark script displays timings
in nanoseconds. The benchmarks were measured on an
<a class="reference external" href="https://ark.intel.com/content/www/us/en/ark/products/76088/intel-core-i7-4960hq-processor-6m-cache-up-to-3-80-ghz.html">Intel® Core™ i7-4960HQ processor</a>
running the macOS 64-bit builds found at
<a class="reference external" href="https://www.python.org/downloads/mac-osx/">python.org</a>.
</section>
<section id="deprecated">
<h2>Deprecated<a class="headerlink" href="#deprecated" title="Permalink to this headline">¶</a></h2>
<ul>
<li>The distutils <code class="docutils literal notranslate">bdist_msi</code> command is now deprecated, use
<code class="docutils literal notranslate">bdist_wheel</code> (wheel packages) instead.
(Contributed by Hugo van Kemenade in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39586">bpo-39586</a>.)</li>
<li>Currently <a class="reference internal" href="../library/math.html#math.factorial" title="math.factorial"><code class="xref py py-func docutils literal notranslate">math.factorial()</code></a> accepts <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a> instances with
non-negative integer values (like <code class="docutils literal notranslate">5.0</code>). It raises a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>
for non-integral and negative floats. It is now deprecated. In future
Python versions it will raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> for all floats.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37315">bpo-37315</a>.)</li>
<li>The <code class="xref py py-mod docutils literal notranslate">parser</code> and <code class="xref py py-mod docutils literal notranslate">symbol</code> modules are deprecated and will be
removed in future versions of Python. For the majority of use cases,
users can leverage the Abstract Syntax Tree (AST) generation and compilation
stage, using the <a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> module.</li>
<li>The Public C API functions <code class="xref c c-func docutils literal notranslate">PyParser_SimpleParseStringFlags()</code>,
<code class="xref c c-func docutils literal notranslate">PyParser_SimpleParseStringFlagsFilename()</code>,
<code class="xref c c-func docutils literal notranslate">PyParser_SimpleParseFileFlags()</code> and <code class="xref c c-func docutils literal notranslate">PyNode_Compile()</code>
are deprecated and will be removed in Python 3.10 together with the old parser.</li>
<li>Using <a class="reference internal" href="../library/constants.html#NotImplemented" title="NotImplemented"><code class="xref py py-data docutils literal notranslate">NotImplemented</code></a> in a boolean context has been deprecated,
as it is almost exclusively the result of incorrect rich comparator
implementations. It will be made a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> in a future version
of Python.
(Contributed by Josh Rosenberg in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35712">bpo-35712</a>.)</li>
<li>The <a class="reference internal" href="../library/random.html#module-random" title="random: Generate pseudo-random numbers with various common distributions."><code class="xref py py-mod docutils literal notranslate">random</code></a> module currently accepts any hashable type as a
possible seed value. Unfortunately, some of those types are not
guaranteed to have a deterministic hash value. After Python 3.9,
the module will restrict its seeds to <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>,
<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>, <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>, and <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a>.</li>
<li>Opening the <a class="reference internal" href="../library/gzip.html#gzip.GzipFile" title="gzip.GzipFile"><code class="xref py py-class docutils literal notranslate">GzipFile</code></a> file for writing without specifying
the mode argument is deprecated. In future Python versions it will always
be opened for reading by default. Specify the mode argument for opening
it for writing and silencing a warning.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=28286">bpo-28286</a>.)</li>
<li>Deprecated the <code class="docutils literal notranslate">split()</code> method of <code class="xref py py-class docutils literal notranslate">_tkinter.TkappType</code> in
favour of the <code class="docutils literal notranslate">splitlist()</code> method which has more consistent and
predicable behavior.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38371">bpo-38371</a>.)</li>
<li>The explicit passing of coroutine objects to <a class="reference internal" href="../library/asyncio-task.html#asyncio.wait" title="asyncio.wait"><code class="xref py py-func docutils literal notranslate">asyncio.wait()</code></a> has been
deprecated and will be removed in version 3.11.
(Contributed by Yury Selivanov and Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34790">bpo-34790</a>.)</li>
<li>binhex4 and hexbin4 standards are now deprecated. The <a class="reference internal" href="../library/binhex.html#module-binhex" title="binhex: Encode and decode files in binhex4 format."><code class="xref py py-mod docutils literal notranslate">binhex</code></a> module
and the following <a class="reference internal" href="../library/binascii.html#module-binascii" title="binascii: Tools for converting between binary and various ASCII-encoded binary representations."><code class="xref py py-mod docutils literal notranslate">binascii</code></a> functions are now deprecated:
<ul class="simple">
<li><a class="reference internal" href="../library/binascii.html#binascii.b2a_hqx" title="binascii.b2a_hqx"><code class="xref py py-func docutils literal notranslate">b2a_hqx()</code></a>, <a class="reference internal" href="../library/binascii.html#binascii.a2b_hqx" title="binascii.a2b_hqx"><code class="xref py py-func docutils literal notranslate">a2b_hqx()</code></a></li>
<li><a class="reference internal" href="../library/binascii.html#binascii.rlecode_hqx" title="binascii.rlecode_hqx"><code class="xref py py-func docutils literal notranslate">rlecode_hqx()</code></a>, <a class="reference internal" href="../library/binascii.html#binascii.rledecode_hqx" title="binascii.rledecode_hqx"><code class="xref py py-func docutils literal notranslate">rledecode_hqx()</code></a></li>
</ul>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39353">bpo-39353</a>.)
</li>
<li><a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> classes <code class="docutils literal notranslate">slice</code>, <code class="docutils literal notranslate">Index</code> and <code class="docutils literal notranslate">ExtSlice</code> are considered deprecated
and will be removed in future Python versions. <code class="docutils literal notranslate">value</code> itself should be
used instead of <code class="docutils literal notranslate">Index(value)</code>. <code class="docutils literal notranslate">Tuple(slices, Load())</code> should be
used instead of <code class="docutils literal notranslate">ExtSlice(slices)</code>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34822">bpo-34822</a>.)</li>
<li><a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> classes <code class="docutils literal notranslate">Suite</code>, <code class="docutils literal notranslate">Param</code>, <code class="docutils literal notranslate">AugLoad</code> and <code class="docutils literal notranslate">AugStore</code>
are considered deprecated and will be removed in future Python versions.
They were not generated by the parser and not accepted by the code
generator in Python 3.
(Contributed by Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39639">bpo-39639</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39969">bpo-39969</a>
and Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39988">bpo-39988</a>.)</li>
<li>The <a class="reference internal" href="../c-api/init.html#c.PyEval_InitThreads" title="PyEval_InitThreads"><code class="xref c c-func docutils literal notranslate">PyEval_InitThreads()</code></a> and <a class="reference internal" href="../c-api/init.html#c.PyEval_ThreadsInitialized" title="PyEval_ThreadsInitialized"><code class="xref c c-func docutils literal notranslate">PyEval_ThreadsInitialized()</code></a>
functions are now deprecated and will be removed in Python 3.11. Calling
<a class="reference internal" href="../c-api/init.html#c.PyEval_InitThreads" title="PyEval_InitThreads"><code class="xref c c-func docutils literal notranslate">PyEval_InitThreads()</code></a> now does nothing. The <a class="reference internal" href="../glossary.html#term-GIL">GIL</a> is initialized
by <a class="reference internal" href="../c-api/init.html#c.Py_Initialize" title="Py_Initialize"><code class="xref c c-func docutils literal notranslate">Py_Initialize()</code></a> since Python 3.7.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39877">bpo-39877</a>.)</li>
<li>Passing <code class="docutils literal notranslate">None</code> as the first argument to the <a class="reference internal" href="../library/shlex.html#shlex.split" title="shlex.split"><code class="xref py py-func docutils literal notranslate">shlex.split()</code></a> function
has been deprecated. (Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33262">bpo-33262</a>.)</li>
<li><a class="reference internal" href="../library/smtpd.html#smtpd.MailmanProxy" title="smtpd.MailmanProxy"><code class="xref py py-func docutils literal notranslate">smtpd.MailmanProxy()</code></a> is now deprecated as it is unusable without
an external module, <code class="docutils literal notranslate">mailman</code>. (Contributed by Samuel Colvin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35800">bpo-35800</a>.)</li>
<li>The <a class="reference internal" href="../library/2to3.html#module-lib2to3" title="lib2to3: The 2to3 library"><code class="xref py py-mod docutils literal notranslate">lib2to3</code></a> module now emits a <a class="reference internal" href="../library/exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><code class="xref py py-exc docutils literal notranslate">PendingDeprecationWarning</code></a>.
Python 3.9 switched to a PEG parser (see <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0617">PEP 617</a>), and Python 3.10 may
include new language syntax that is not parsable by lib2to3’s LL(1) parser.
The <code class="docutils literal notranslate">lib2to3</code> module may be removed from the standard library in a future
Python version. Consider third-party alternatives such as <a class="reference external" href="https://libcst.readthedocs.io/">LibCST</a> or
<a class="reference external" href="https://parso.readthedocs.io/">parso</a>.
(Contributed by Carl Meyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40360">bpo-40360</a>.)</li>
<li>The random parameter of <a class="reference internal" href="../library/random.html#random.shuffle" title="random.shuffle"><code class="xref py py-func docutils literal notranslate">random.shuffle()</code></a> has been deprecated.
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40465">bpo-40465</a>)</li>
</ul>
</section>
<section id="removed">
<h2>Removed<a class="headerlink" href="#removed" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>The erroneous version at <code class="xref py py-data docutils literal notranslate">unittest.mock.__version__</code> has been removed.</li>
<li><a class="reference internal" href="../library/nntplib.html#nntplib.NNTP" title="nntplib.NNTP"><code class="xref py py-class docutils literal notranslate">nntplib.NNTP</code></a>: <code class="docutils literal notranslate">xpath()</code> and <code class="docutils literal notranslate">xgtitle()</code> methods have been removed.
These methods are deprecated since Python 3.3. Generally, these extensions
are not supported or not enabled by NNTP server administrators.
For <code class="docutils literal notranslate">xgtitle()</code>, please use <a class="reference internal" href="../library/nntplib.html#nntplib.NNTP.descriptions" title="nntplib.NNTP.descriptions"><code class="xref py py-meth docutils literal notranslate">nntplib.NNTP.descriptions()</code></a> or
<a class="reference internal" href="../library/nntplib.html#nntplib.NNTP.description" title="nntplib.NNTP.description"><code class="xref py py-meth docutils literal notranslate">nntplib.NNTP.description()</code></a> instead.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39366">bpo-39366</a>.)</li>
<li><a class="reference internal" href="../library/array.html#array.array" title="array.array"><code class="xref py py-class docutils literal notranslate">array.array</code></a>: <code class="docutils literal notranslate">tostring()</code> and <code class="docutils literal notranslate">fromstring()</code> methods have been
removed. They were aliases to <code class="docutils literal notranslate">tobytes()</code> and <code class="docutils literal notranslate">frombytes()</code>, deprecated
since Python 3.2.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38916">bpo-38916</a>.)</li>
<li>The undocumented <code class="docutils literal notranslate">sys.callstats()</code> function has been removed. Since Python
3.7, it was deprecated and always returned <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>. It required a special
build option <code class="docutils literal notranslate">CALL_PROFILE</code> which was already removed in Python 3.7.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37414">bpo-37414</a>.)</li>
<li>The <code class="docutils literal notranslate">sys.getcheckinterval()</code> and <code class="docutils literal notranslate">sys.setcheckinterval()</code> functions have
been removed. They were deprecated since Python 3.2. Use
<a class="reference internal" href="../library/sys.html#sys.getswitchinterval" title="sys.getswitchinterval"><code class="xref py py-func docutils literal notranslate">sys.getswitchinterval()</code></a> and <a class="reference internal" href="../library/sys.html#sys.setswitchinterval" title="sys.setswitchinterval"><code class="xref py py-func docutils literal notranslate">sys.setswitchinterval()</code></a> instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37392">bpo-37392</a>.)</li>
<li>The C function <code class="docutils literal notranslate">PyImport_Cleanup()</code> has been removed. It was documented as:
“Empty the module table. For internal use only.”
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36710">bpo-36710</a>.)</li>
<li><code class="docutils literal notranslate">_dummy_thread</code> and <code class="docutils literal notranslate">dummy_threading</code> modules have been removed. These
modules were deprecated since Python 3.7 which requires threading support.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37312">bpo-37312</a>.)</li>
<li><code class="docutils literal notranslate">aifc.openfp()</code> alias to <code class="docutils literal notranslate">aifc.open()</code>, <code class="docutils literal notranslate">sunau.openfp()</code> alias to
<code class="docutils literal notranslate">sunau.open()</code>, and <code class="docutils literal notranslate">wave.openfp()</code> alias to <a class="reference internal" href="../library/wave.html#wave.open" title="wave.open"><code class="xref py py-func docutils literal notranslate">wave.open()</code></a> have been
removed. They were deprecated since Python 3.7.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37320">bpo-37320</a>.)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">isAlive()</code> method of <a class="reference internal" href="../library/threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate">threading.Thread</code></a>
has been removed. It was deprecated since Python 3.8.
Use <a class="reference internal" href="../library/threading.html#threading.Thread.is_alive" title="threading.Thread.is_alive"><code class="xref py py-meth docutils literal notranslate">is_alive()</code></a> instead.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37804">bpo-37804</a>.)</li>
<li>Methods <code class="docutils literal notranslate">getchildren()</code> and <code class="docutils literal notranslate">getiterator()</code> of classes
<a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.ElementTree" title="xml.etree.ElementTree.ElementTree"><code class="xref py py-class docutils literal notranslate">ElementTree</code></a> and
<a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.Element" title="xml.etree.ElementTree.Element"><code class="xref py py-class docutils literal notranslate">Element</code></a> in the <a class="reference internal" href="../library/xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal notranslate">ElementTree</code></a>
module have been removed. They were deprecated in Python 3.2.
Use <code class="docutils literal notranslate">iter(x)</code> or <code class="docutils literal notranslate">list(x)</code> instead of <code class="docutils literal notranslate">x.getchildren()</code> and
<code class="docutils literal notranslate">x.iter()</code> or <code class="docutils literal notranslate">list(x.iter())</code> instead of <code class="docutils literal notranslate">x.getiterator()</code>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36543">bpo-36543</a>.)</li>
<li>The old <a class="reference internal" href="../library/plistlib.html#module-plistlib" title="plistlib: Generate and parse Apple plist files."><code class="xref py py-mod docutils literal notranslate">plistlib</code></a> API has been removed, it was deprecated since Python
3.4. Use the <a class="reference internal" href="../library/plistlib.html#plistlib.load" title="plistlib.load"><code class="xref py py-func docutils literal notranslate">load()</code></a>, <a class="reference internal" href="../library/plistlib.html#plistlib.loads" title="plistlib.loads"><code class="xref py py-func docutils literal notranslate">loads()</code></a>, <a class="reference internal" href="../library/plistlib.html#plistlib.dump" title="plistlib.dump"><code class="xref py py-func docutils literal notranslate">dump()</code></a>, and
<a class="reference internal" href="../library/plistlib.html#plistlib.dumps" title="plistlib.dumps"><code class="xref py py-func docutils literal notranslate">dumps()</code></a> functions. Additionally, the use_builtin_types parameter was
removed, standard <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> objects are always used instead.
(Contributed by Jon Janzen in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36409">bpo-36409</a>.)</li>
<li>The C function <code class="docutils literal notranslate">PyGen_NeedsFinalizing</code> has been removed. It was not
documented, tested, or used anywhere within CPython after the implementation
of <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0442">PEP 442</a>. Patch by Joannah Nanjekye.
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=15088">bpo-15088</a>)</li>
<li><code class="docutils literal notranslate">base64.encodestring()</code> and <code class="docutils literal notranslate">base64.decodestring()</code>, aliases deprecated
since Python 3.1, have been removed: use <a class="reference internal" href="../library/base64.html#base64.encodebytes" title="base64.encodebytes"><code class="xref py py-func docutils literal notranslate">base64.encodebytes()</code></a> and
<a class="reference internal" href="../library/base64.html#base64.decodebytes" title="base64.decodebytes"><code class="xref py py-func docutils literal notranslate">base64.decodebytes()</code></a> instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39351">bpo-39351</a>.)</li>
<li><code class="docutils literal notranslate">fractions.gcd()</code> function has been removed, it was deprecated since Python
3.5 (<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=22486">bpo-22486</a>): use <a class="reference internal" href="../library/math.html#math.gcd" title="math.gcd"><code class="xref py py-func docutils literal notranslate">math.gcd()</code></a> instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39350">bpo-39350</a>.)</li>
<li>The buffering parameter of <a class="reference internal" href="../library/bz2.html#bz2.BZ2File" title="bz2.BZ2File"><code class="xref py py-class docutils literal notranslate">bz2.BZ2File</code></a> has been removed. Since
Python 3.0, it was ignored and using it emitted a <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a>.
Pass an open file object to control how the file is opened.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39357">bpo-39357</a>.)</li>
<li>The encoding parameter of <a class="reference internal" href="../library/json.html#json.loads" title="json.loads"><code class="xref py py-func docutils literal notranslate">json.loads()</code></a> has been removed.
As of Python 3.1, it was deprecated and ignored; using it has emitted a
<a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> since Python 3.8.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39377">bpo-39377</a>)</li>
<li><code class="docutils literal notranslate">with (await asyncio.lock):</code> and <code class="docutils literal notranslate">with (yield from asyncio.lock):</code> statements are
not longer supported, use <code class="docutils literal notranslate">async with lock</code> instead. The same is correct for
<code class="docutils literal notranslate">asyncio.Condition</code> and <code class="docutils literal notranslate">asyncio.Semaphore</code>.
(Contributed by Andrew Svetlov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34793">bpo-34793</a>.)</li>
<li>The <code class="xref py py-func docutils literal notranslate">sys.getcounts()</code> function, the <code class="docutils literal notranslate">-X showalloccount</code> command line
option and the <code class="docutils literal notranslate">show_alloc_count</code> field of the C structure
<a class="reference internal" href="../c-api/init_config.html#c.PyConfig" title="PyConfig"><code class="xref c c-type docutils literal notranslate">PyConfig</code></a> have been removed. They required a special Python build by
defining <code class="docutils literal notranslate">COUNT_ALLOCS</code> macro.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39489">bpo-39489</a>.)</li>
<li>The <code class="docutils literal notranslate">_field_types</code> attribute of the <a class="reference internal" href="../library/typing.html#typing.NamedTuple" title="typing.NamedTuple"><code class="xref py py-class docutils literal notranslate">typing.NamedTuple</code></a> class
has been removed. It was deprecated since Python 3.8. Use
the <code class="docutils literal notranslate">__annotations__</code> attribute instead.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40182">bpo-40182</a>.)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">symtable.SymbolTable.has_exec()</code> method has been removed. It was
deprecated since 2006, and only returning <code class="docutils literal notranslate">False</code> when it’s called.
(Contributed by Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40208">bpo-40208</a>)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">asyncio.Task.current_task()</code> and <code class="xref py py-meth docutils literal notranslate">asyncio.Task.all_tasks()</code>
have been removed. They were deprecated since Python 3.7 and you can use
<a class="reference internal" href="../library/asyncio-task.html#asyncio.current_task" title="asyncio.current_task"><code class="xref py py-func docutils literal notranslate">asyncio.current_task()</code></a> and <a class="reference internal" href="../library/asyncio-task.html#asyncio.all_tasks" title="asyncio.all_tasks"><code class="xref py py-func docutils literal notranslate">asyncio.all_tasks()</code></a> instead.
(Contributed by Rémi Lapeyre in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40967">bpo-40967</a>)</li>
<li>The <code class="docutils literal notranslate">unescape()</code> method in the <a class="reference internal" href="../library/html.parser.html#html.parser.HTMLParser" title="html.parser.HTMLParser"><code class="xref py py-class docutils literal notranslate">html.parser.HTMLParser</code></a> class
has been removed (it was deprecated since Python 3.4). <a class="reference internal" href="../library/html.html#html.unescape" title="html.unescape"><code class="xref py py-func docutils literal notranslate">html.unescape()</code></a>
should be used for converting character references to the corresponding
unicode characters.</li>
</ul>
</section>
<section id="porting-to-python-3-9">
<h2>Porting to Python 3.9<a class="headerlink" href="#porting-to-python-3-9" title="Permalink to this headline">¶</a></h2>
This section lists previously described changes and other bugfixes
that may require changes to your code.
<section id="changes-in-the-python-api">
<h3>Changes in the Python API<a class="headerlink" href="#changes-in-the-python-api" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><a class="reference internal" href="../library/functions.html#import__" title="__import__"><code class="xref py py-func docutils literal notranslate">__import__()</code></a> and <a class="reference internal" href="../library/importlib.html#importlib.util.resolve_name" title="importlib.util.resolve_name"><code class="xref py py-func docutils literal notranslate">importlib.util.resolve_name()</code></a> now raise
<a class="reference internal" href="../library/exceptions.html#ImportError" title="ImportError"><code class="xref py py-exc docutils literal notranslate">ImportError</code></a> where it previously raised <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>. Callers
catching the specific exception type and supporting both Python 3.9 and
earlier versions will need to catch both using <code class="docutils literal notranslate">except (ImportError, ValueError):</code>.</li>
<li>The <a class="reference internal" href="../library/venv.html#module-venv" title="venv: Creation of virtual environments."><code class="xref py py-mod docutils literal notranslate">venv</code></a> activation scripts no longer special-case when
<code class="docutils literal notranslate">__VENV_PROMPT__</code> is set to <code class="docutils literal notranslate">&quot;&quot;</code>.</li>
<li>The <a class="reference internal" href="../library/select.html#select.epoll.unregister" title="select.epoll.unregister"><code class="xref py py-meth docutils literal notranslate">select.epoll.unregister()</code></a> method no longer ignores the
<a class="reference internal" href="../library/errno.html#errno.EBADF" title="errno.EBADF"><code class="xref py py-data docutils literal notranslate">EBADF</code></a> error.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39239">bpo-39239</a>.)</li>
<li>The compresslevel parameter of <a class="reference internal" href="../library/bz2.html#bz2.BZ2File" title="bz2.BZ2File"><code class="xref py py-class docutils literal notranslate">bz2.BZ2File</code></a> became keyword-only,
since the buffering parameter has been removed.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39357">bpo-39357</a>.)</li>
<li>Simplified AST for subscription. Simple indices will be represented by
their value, extended slices will be represented as tuples.
<code class="docutils literal notranslate">Index(value)</code> will return a <code class="docutils literal notranslate">value</code> itself, <code class="docutils literal notranslate">ExtSlice(slices)</code>
will return <code class="docutils literal notranslate">Tuple(slices, Load())</code>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34822">bpo-34822</a>.)</li>
<li>The <a class="reference internal" href="../library/importlib.html#module-importlib" title="importlib: The implementation of the import machinery."><code class="xref py py-mod docutils literal notranslate">importlib</code></a> module now ignores the <a class="reference internal" href="../using/cmdline.html#envvar-PYTHONCASEOK"><code class="xref std std-envvar docutils literal notranslate">PYTHONCASEOK</code></a>
environment variable when the <a class="reference internal" href="../using/cmdline.html#cmdoption-E"><code class="xref std std-option docutils literal notranslate">-E</code></a> or <a class="reference internal" href="../using/cmdline.html#cmdoption-I"><code class="xref std std-option docutils literal notranslate">-I</code></a> command line
options are being used.</li>
<li>The encoding parameter has been added to the classes <a class="reference internal" href="../library/ftplib.html#ftplib.FTP" title="ftplib.FTP"><code class="xref py py-class docutils literal notranslate">ftplib.FTP</code></a> and
<a class="reference internal" href="../library/ftplib.html#ftplib.FTP_TLS" title="ftplib.FTP_TLS"><code class="xref py py-class docutils literal notranslate">ftplib.FTP_TLS</code></a> as a keyword-only parameter, and the default encoding
is changed from Latin-1 to UTF-8 to follow <a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc2640.html">RFC 2640</a>.</li>
<li><a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.shutdown_default_executor" title="asyncio.loop.shutdown_default_executor"><code class="xref py py-meth docutils literal notranslate">asyncio.loop.shutdown_default_executor()</code></a> has been added to
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.AbstractEventLoop" title="asyncio.AbstractEventLoop"><code class="xref py py-class docutils literal notranslate">AbstractEventLoop</code></a>, meaning alternative event loops that
inherit from it should have this method defined.
(Contributed by Kyle Stanley in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34037">bpo-34037</a>.)</li>
<li>The constant values of future flags in the <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate">__future__</code></a> module
is updated in order to prevent collision with compiler flags. Previously
<code class="docutils literal notranslate">PyCF_ALLOW_TOP_LEVEL_AWAIT</code> was clashing with <code class="docutils literal notranslate">CO_FUTURE_DIVISION</code>.
(Contributed by Batuhan Taskaya in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39562">bpo-39562</a>)</li>
<li><code class="docutils literal notranslate">array('u')</code> now uses <code class="docutils literal notranslate">wchar_t</code> as C type instead of <code class="docutils literal notranslate">Py_UNICODE</code>.
This change doesn’t affect to its behavior because <code class="docutils literal notranslate">Py_UNICODE</code> is alias
of <code class="docutils literal notranslate">wchar_t</code> since Python 3.3.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34538">bpo-34538</a>.)</li>
<li>The <a class="reference internal" href="../library/logging.html#logging.getLogger" title="logging.getLogger"><code class="xref py py-func docutils literal notranslate">logging.getLogger()</code></a> API now returns the root logger when passed
the name <code class="docutils literal notranslate">'root'</code>, whereas previously it returned a non-root logger named
<code class="docutils literal notranslate">'root'</code>. This could affect cases where user code explicitly wants a
non-root logger named <code class="docutils literal notranslate">'root'</code>, or instantiates a logger using
<code class="docutils literal notranslate">logging.getLogger(__name__)</code> in some top-level module called <code class="docutils literal notranslate">'root.py'</code>.
(Contributed by Vinay Sajip in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37742">bpo-37742</a>.)</li>
<li>Division handling of <a class="reference internal" href="../library/pathlib.html#pathlib.PurePath" title="pathlib.PurePath"><code class="xref py py-class docutils literal notranslate">PurePath</code></a> now returns <code class="docutils literal notranslate">NotImplemented</code>
instead of raising a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> when passed something other than an
instance of <code class="docutils literal notranslate">str</code> or <a class="reference internal" href="../library/pathlib.html#pathlib.PurePath" title="pathlib.PurePath"><code class="xref py py-class docutils literal notranslate">PurePath</code></a>. This allows creating
compatible classes that don’t inherit from those mentioned types.
(Contributed by Roger Aiudi in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34775">bpo-34775</a>).</li>
<li>Starting with Python 3.9.5 the <a class="reference internal" href="../library/ipaddress.html#module-ipaddress" title="ipaddress: IPv4/IPv6 manipulation library."><code class="xref py py-mod docutils literal notranslate">ipaddress</code></a> module no longer
accepts any leading zeros in IPv4 address strings. Leading zeros are
ambiguous and interpreted as octal notation by some libraries. For example
the legacy function <a class="reference internal" href="../library/socket.html#socket.inet_aton" title="socket.inet_aton"><code class="xref py py-func docutils literal notranslate">socket.inet_aton()</code></a> treats leading zeros as octal
notatation. glibc implementation of modern <a class="reference internal" href="../library/socket.html#socket.inet_pton" title="socket.inet_pton"><code class="xref py py-func docutils literal notranslate">inet_pton()</code></a> does
not accept any leading zeros.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36384">bpo-36384</a>).</li>
<li><a class="reference internal" href="../library/codecs.html#codecs.lookup" title="codecs.lookup"><code class="xref py py-func docutils literal notranslate">codecs.lookup()</code></a> now normalizes the encoding name the same way as
<code class="xref py py-func docutils literal notranslate">encodings.normalize_encoding()</code>, except that <a class="reference internal" href="../library/codecs.html#codecs.lookup" title="codecs.lookup"><code class="xref py py-func docutils literal notranslate">codecs.lookup()</code></a> also
converts the name to lower case. For example, <code class="docutils literal notranslate">&quot;latex+latin1&quot;</code> encoding
name is now normalized to <code class="docutils literal notranslate">&quot;latex_latin1&quot;</code>.
(Contributed by Jordon Xu in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37751">bpo-37751</a>.)</li>
</ul>
</section>
<section id="changes-in-the-c-api">
<h3>Changes in the C API<a class="headerlink" href="#changes-in-the-c-api" title="Permalink to this headline">¶</a></h3>
<ul>
<li>Instances of <a class="reference internal" href="../c-api/typeobj.html#heap-types">heap-allocated types</a> (such as those created with
<a class="reference internal" href="../c-api/type.html#c.PyType_FromSpec" title="PyType_FromSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromSpec()</code></a> and similar APIs) hold a reference to their type
object since Python 3.8. As indicated in the “Changes in the C API” of Python
3.8, for the vast majority of cases, there should be no side effect but for
types that have a custom <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> function,
ensure that all custom <code class="docutils literal notranslate">tp_traverse</code> functions of heap-allocated types
visit the object’s type.
<blockquote>
<div>Example:
<div class="highlight-c notranslate"><div class="highlight"><pre>int
foo_traverse(foo_struct *self, visitproc visit, void *arg) {
// Rest of the traverse function
#if PY_VERSION_HEX &gt;= 0x03090000
 // This was not needed before Python 3.9 (Python issue 35810 and 40217)
 Py_VISIT(Py_TYPE(self));
#endif
}
</pre></div>
</div>
</div></blockquote>
If your traverse function delegates to <code class="docutils literal notranslate">tp_traverse</code> of its base class
(or another type), ensure that <code class="docutils literal notranslate">Py_TYPE(self)</code> is visited only once.
Note that only <a class="reference internal" href="../c-api/typeobj.html#heap-types">heap type</a> are expected to visit the type
in <code class="docutils literal notranslate">tp_traverse</code>.
<blockquote>
<div>For example, if your <code class="docutils literal notranslate">tp_traverse</code> function includes:
<div class="highlight-c notranslate"><div class="highlight"><pre>base-&gt;tp_traverse(self, visit, arg)
</pre></div>
</div>
then add:
<div class="highlight-c notranslate"><div class="highlight"><pre>#if PY_VERSION_HEX &gt;= 0x03090000
 // This was not needed before Python 3.9 (bpo-35810 and bpo-40217)
 if (base-&gt;tp_flags &amp; Py_TPFLAGS_HEAPTYPE) {
 // a heap type&#39;s tp_traverse already visited Py_TYPE(self)
 } else {
 Py_VISIT(Py_TYPE(self));
 }
#else
</pre></div>
</div>
</div></blockquote>
(See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35810">bpo-35810</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40217">bpo-40217</a> for more information.)
</li>
<li>The functions <code class="docutils literal notranslate">PyEval_CallObject</code>, <code class="docutils literal notranslate">PyEval_CallFunction</code>,
<code class="docutils literal notranslate">PyEval_CallMethod</code> and <code class="docutils literal notranslate">PyEval_CallObjectWithKeywords</code> are deprecated.
Use <a class="reference internal" href="../c-api/call.html#c.PyObject_Call" title="PyObject_Call"><code class="xref c c-func docutils literal notranslate">PyObject_Call()</code></a> and its variants instead.
(See more details in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29548">bpo-29548</a>.)</li>
</ul>
</section>
<section id="cpython-bytecode-changes">
<h3>CPython bytecode changes<a class="headerlink" href="#cpython-bytecode-changes" title="Permalink to this headline">¶</a></h3>
<ul>
<li>The <a class="reference internal" href="../library/dis.html#opcode-LOAD_ASSERTION_ERROR"><code class="xref std std-opcode docutils literal notranslate">LOAD_ASSERTION_ERROR</code></a> opcode was added for handling the
<a class="reference internal" href="../reference/simple_stmts.html#assert"><code class="xref std std-keyword docutils literal notranslate">assert</code></a> statement. Previously, the assert statement would not work
correctly if the <a class="reference internal" href="../library/exceptions.html#AssertionError" title="AssertionError"><code class="xref py py-exc docutils literal notranslate">AssertionError</code></a> exception was being shadowed.
(Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34880">bpo-34880</a>.)</li>
<li>The <a class="reference internal" href="../library/dis.html#opcode-COMPARE_OP"><code class="xref std std-opcode docutils literal notranslate">COMPARE_OP</code></a> opcode was split into four distinct instructions:
<ul class="simple">
<li><code class="docutils literal notranslate">COMPARE_OP</code> for rich comparisons</li>
<li><code class="docutils literal notranslate">IS_OP</code> for ‘is’ and ‘is not’ tests</li>
<li><code class="docutils literal notranslate">CONTAINS_OP</code> for ‘in’ and ‘not in’ tests</li>
<li><code class="docutils literal notranslate">JUMP_IF_NOT_EXC_MATCH</code> for checking exceptions in ‘try-except’
statements.</li>
</ul>
(Contributed by Mark Shannon in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39156">bpo-39156</a>.)
</li>
</ul>
</section>
</section>
<section id="build-changes">
<h2>Build Changes<a class="headerlink" href="#build-changes" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>Added <code class="docutils literal notranslate">--with-platlibdir</code> option to the <code class="docutils literal notranslate">configure</code> script: name of the
platform-specific library directory, stored in the new <a class="reference internal" href="../library/sys.html#sys.platlibdir" title="sys.platlibdir"><code class="xref py py-attr docutils literal notranslate">sys.platlibdir</code></a>
attribute. See <a class="reference internal" href="../library/sys.html#sys.platlibdir" title="sys.platlibdir"><code class="xref py py-attr docutils literal notranslate">sys.platlibdir</code></a> attribute for more information.
(Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis
and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1294959">bpo-1294959</a>.)</li>
<li>The <code class="docutils literal notranslate">COUNT_ALLOCS</code> special build macro has been removed.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39489">bpo-39489</a>.)</li>
<li>On non-Windows platforms, the <code class="xref c c-func docutils literal notranslate">setenv()</code> and <code class="xref c c-func docutils literal notranslate">unsetenv()</code>
functions are now required to build Python.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39395">bpo-39395</a>.)</li>
<li>On non-Windows platforms, creating <code class="docutils literal notranslate">bdist_wininst</code> installers is now
officially unsupported. (See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=10945">bpo-10945</a> for more details.)</li>
<li>When building Python on macOS from source, <code class="docutils literal notranslate">_tkinter</code> now links with
non-system Tcl and Tk frameworks if they are installed in
<code class="docutils literal notranslate">/Library/Frameworks</code>, as had been the case on older releases
of macOS. If a macOS SDK is explicitly configured, by using
<a class="reference internal" href="../using/configure.html#cmdoption-1"><code class="xref std std-option docutils literal notranslate">--enable-universalsdk</code></a> or <code class="docutils literal notranslate">-isysroot</code>, only the SDK itself is
searched. The default behavior can still be overridden with
<a class="reference internal" href="../using/configure.html#cmdoption-with-tcltk-includes"><code class="xref std std-option docutils literal notranslate">--with-tcltk-includes</code></a> and <a class="reference internal" href="../using/configure.html#cmdoption-with-tcltk-libs"><code class="xref std std-option docutils literal notranslate">--with-tcltk-libs</code></a>.
(Contributed by Ned Deily in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34956">bpo-34956</a>.)</li>
<li>Python can now be built for Windows 10 ARM64.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33125">bpo-33125</a>.)</li>
<li>Some individual tests are now skipped when <code class="docutils literal notranslate">--pgo</code> is used. The tests
in question increased the PGO task time significantly and likely
didn’t help improve optimization of the final executable. This
speeds up the task by a factor of about 15x. Running the full unit test
suite is slow. This change may result in a slightly less optimized build
since not as many code branches will be executed. If you are willing to
wait for the much slower build, the old behavior can be restored using
<code class="docutils literal notranslate">./configure [..] PROFILE_TASK=&quot;-m test --pgo-extended&quot;</code>. We make no
guarantees as to which PGO task set produces a faster build. Users who care
should run their own relevant benchmarks as results can depend on the
environment, workload, and compiler tool chain.
(See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36044">bpo-36044</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37707">bpo-37707</a> for more details.)</li>
</ul>
</section>
<section id="c-api-changes">
<h2>C API Changes<a class="headerlink" href="#c-api-changes" title="Permalink to this headline">¶</a></h2>
<section id="id1">
<h3>New Features<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
<ul>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0573">PEP 573</a>: Added <a class="reference internal" href="../c-api/type.html#c.PyType_FromModuleAndSpec" title="PyType_FromModuleAndSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromModuleAndSpec()</code></a> to associate
a module with a class; <a class="reference internal" href="../c-api/type.html#c.PyType_GetModule" title="PyType_GetModule"><code class="xref c c-func docutils literal notranslate">PyType_GetModule()</code></a> and
<a class="reference internal" href="../c-api/type.html#c.PyType_GetModuleState" title="PyType_GetModuleState"><code class="xref c c-func docutils literal notranslate">PyType_GetModuleState()</code></a> to retrieve the module and its state; and
<a class="reference internal" href="../c-api/structures.html#c.PyCMethod" title="PyCMethod"><code class="xref c c-data docutils literal notranslate">PyCMethod</code></a> and <code class="xref c c-data docutils literal notranslate">METH_METHOD</code> to allow a method to
access the class it was defined in.
(Contributed by Marcel Plch and Petr Viktorin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38787">bpo-38787</a>.)</li>
<li>Added <a class="reference internal" href="../c-api/reflection.html#c.PyFrame_GetCode" title="PyFrame_GetCode"><code class="xref c c-func docutils literal notranslate">PyFrame_GetCode()</code></a> function: get a frame code.
Added <a class="reference internal" href="../c-api/reflection.html#c.PyFrame_GetBack" title="PyFrame_GetBack"><code class="xref c c-func docutils literal notranslate">PyFrame_GetBack()</code></a> function: get the frame next outer frame.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40421">bpo-40421</a>.)</li>
<li>Added <a class="reference internal" href="../c-api/reflection.html#c.PyFrame_GetLineNumber" title="PyFrame_GetLineNumber"><code class="xref c c-func docutils literal notranslate">PyFrame_GetLineNumber()</code></a> to the limited C API.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40421">bpo-40421</a>.)</li>
<li>Added <a class="reference internal" href="../c-api/init.html#c.PyThreadState_GetInterpreter" title="PyThreadState_GetInterpreter"><code class="xref c c-func docutils literal notranslate">PyThreadState_GetInterpreter()</code></a> and
<a class="reference internal" href="../c-api/init.html#c.PyInterpreterState_Get" title="PyInterpreterState_Get"><code class="xref c c-func docutils literal notranslate">PyInterpreterState_Get()</code></a> functions to get the interpreter.
Added <a class="reference internal" href="../c-api/init.html#c.PyThreadState_GetFrame" title="PyThreadState_GetFrame"><code class="xref c c-func docutils literal notranslate">PyThreadState_GetFrame()</code></a> function to get the current frame of a
Python thread state.
Added <a class="reference internal" href="../c-api/init.html#c.PyThreadState_GetID" title="PyThreadState_GetID"><code class="xref c c-func docutils literal notranslate">PyThreadState_GetID()</code></a> function: get the unique identifier of a
Python thread state.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39947">bpo-39947</a>.)</li>
<li>Added a new public <a class="reference internal" href="../c-api/call.html#c.PyObject_CallNoArgs" title="PyObject_CallNoArgs"><code class="xref c c-func docutils literal notranslate">PyObject_CallNoArgs()</code></a> function to the C API, which
calls a callable Python object without any arguments. It is the most efficient
way to call a callable Python object without any argument.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37194">bpo-37194</a>.)</li>
<li>Changes in the limited C API (if <code class="docutils literal notranslate">Py_LIMITED_API</code> macro is defined):
<ul class="simple">
<li>Provide <a class="reference internal" href="../c-api/exceptions.html#c.Py_EnterRecursiveCall" title="Py_EnterRecursiveCall"><code class="xref c c-func docutils literal notranslate">Py_EnterRecursiveCall()</code></a> and <a class="reference internal" href="../c-api/exceptions.html#c.Py_LeaveRecursiveCall" title="Py_LeaveRecursiveCall"><code class="xref c c-func docutils literal notranslate">Py_LeaveRecursiveCall()</code></a>
as regular functions for the limited API. Previously, there were defined as
macros, but these macros didn’t compile with the limited C API which cannot
access <code class="docutils literal notranslate">PyThreadState.recursion_depth</code> field (the structure is opaque in
the limited C API).</li>
<li><code class="docutils literal notranslate">PyObject_INIT()</code> and <code class="docutils literal notranslate">PyObject_INIT_VAR()</code> become regular “opaque”
function to hide implementation details.</li>
</ul>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38644">bpo-38644</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39542">bpo-39542</a>.)
</li>
<li>The <a class="reference internal" href="../c-api/module.html#c.PyModule_AddType" title="PyModule_AddType"><code class="xref c c-func docutils literal notranslate">PyModule_AddType()</code></a> function is added to help adding a type
to a module.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40024">bpo-40024</a>.)</li>
<li>Added the functions <a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_IsTracked" title="PyObject_GC_IsTracked"><code class="xref c c-func docutils literal notranslate">PyObject_GC_IsTracked()</code></a> and
<a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_IsFinalized" title="PyObject_GC_IsFinalized"><code class="xref c c-func docutils literal notranslate">PyObject_GC_IsFinalized()</code></a> to the public API to allow to query if
Python objects are being currently tracked or have been already finalized by
the garbage collector respectively.
(Contributed by Pablo Galindo Salgado in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40241">bpo-40241</a>.)</li>
<li>Added <code class="xref c c-func docutils literal notranslate">_PyObject_FunctionStr()</code> to get a user-friendly string
representation of a function-like object.
(Patch by Jeroen Demeyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37645">bpo-37645</a>.)</li>
<li>Added <a class="reference internal" href="../c-api/call.html#c.PyObject_CallOneArg" title="PyObject_CallOneArg"><code class="xref c c-func docutils literal notranslate">PyObject_CallOneArg()</code></a> for calling an object with one
positional argument
(Patch by Jeroen Demeyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37483">bpo-37483</a>.)</li>
</ul>
</section>
<section id="id2">
<h3>Porting to Python 3.9<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h3>
<ul>
<li><code class="docutils literal notranslate">PyInterpreterState.eval_frame</code> (<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0523">PEP 523</a>) now requires a new mandatory
tstate parameter (<code class="docutils literal notranslate">PyThreadState*</code>).
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38500">bpo-38500</a>.)</li>
<li>Extension modules: <a class="reference internal" href="../c-api/module.html#c.PyModuleDef.m_traverse" title="PyModuleDef.m_traverse"><code class="xref c c-member docutils literal notranslate">m_traverse</code></a>,
<a class="reference internal" href="../c-api/module.html#c.PyModuleDef.m_clear" title="PyModuleDef.m_clear"><code class="xref c c-member docutils literal notranslate">m_clear</code></a> and <a class="reference internal" href="../c-api/module.html#c.PyModuleDef.m_free" title="PyModuleDef.m_free"><code class="xref c c-member docutils literal notranslate">m_free</code></a>
functions of <a class="reference internal" href="../c-api/module.html#c.PyModuleDef" title="PyModuleDef"><code class="xref c c-type docutils literal notranslate">PyModuleDef</code></a> are no longer called if the module state
was requested but is not allocated yet. This is the case immediately after
the module is created and before the module is executed
(<a class="reference internal" href="../c-api/module.html#c.Py_mod_exec" title="Py_mod_exec"><code class="xref c c-data docutils literal notranslate">Py_mod_exec</code></a> function). More precisely, these functions are not called
if <a class="reference internal" href="../c-api/module.html#c.PyModuleDef.m_size" title="PyModuleDef.m_size"><code class="xref c c-member docutils literal notranslate">m_size</code></a> is greater than 0 and the module state (as
returned by <a class="reference internal" href="../c-api/module.html#c.PyModule_GetState" title="PyModule_GetState"><code class="xref c c-func docutils literal notranslate">PyModule_GetState()</code></a>) is <code class="docutils literal notranslate">NULL</code>.
Extension modules without module state (<code class="docutils literal notranslate">m_size &lt;= 0</code>) are not affected.
</li>
<li>If <a class="reference internal" href="../c-api/init.html#c.Py_AddPendingCall" title="Py_AddPendingCall"><code class="xref c c-func docutils literal notranslate">Py_AddPendingCall()</code></a> is called in a subinterpreter, the function is
now scheduled to be called from the subinterpreter, rather than being called
from the main interpreter. Each subinterpreter now has its own list of
scheduled calls.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39984">bpo-39984</a>.)</li>
<li>The Windows registry is no longer used to initialize <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate">sys.path</code></a> when
the <code class="docutils literal notranslate">-E</code> option is used (if <a class="reference internal" href="../c-api/init_config.html#c.PyConfig.use_environment" title="PyConfig.use_environment"><code class="xref c c-member docutils literal notranslate">PyConfig.use_environment</code></a> is set to
<code class="docutils literal notranslate">0</code>). This is significant when embedding Python on Windows.
(Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=8901">bpo-8901</a>.)</li>
<li>The global variable <a class="reference internal" href="../c-api/tuple.html#c.PyStructSequence_UnnamedField" title="PyStructSequence_UnnamedField"><code class="xref c c-data docutils literal notranslate">PyStructSequence_UnnamedField</code></a> is now a constant
and refers to a constant string.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38650">bpo-38650</a>.)</li>
<li>The <code class="xref c c-type docutils literal notranslate">PyGC_Head</code> structure is now opaque. It is only defined in the
internal C API (<code class="docutils literal notranslate">pycore_gc.h</code>).
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40241">bpo-40241</a>.)</li>
<li>The <code class="docutils literal notranslate">Py_UNICODE_COPY</code>, <code class="docutils literal notranslate">Py_UNICODE_FILL</code>, <code class="docutils literal notranslate">PyUnicode_WSTR_LENGTH</code>,
<a class="reference internal" href="../c-api/unicode.html#c.PyUnicode_FromUnicode" title="PyUnicode_FromUnicode"><code class="xref c c-func docutils literal notranslate">PyUnicode_FromUnicode()</code></a>, <a class="reference internal" href="../c-api/unicode.html#c.PyUnicode_AsUnicode" title="PyUnicode_AsUnicode"><code class="xref c c-func docutils literal notranslate">PyUnicode_AsUnicode()</code></a>,
<code class="docutils literal notranslate">_PyUnicode_AsUnicode</code>, and <a class="reference internal" href="../c-api/unicode.html#c.PyUnicode_AsUnicodeAndSize" title="PyUnicode_AsUnicodeAndSize"><code class="xref c c-func docutils literal notranslate">PyUnicode_AsUnicodeAndSize()</code></a> are
marked as deprecated in C. They have been deprecated by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0393">PEP 393</a> since
Python 3.3.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36346">bpo-36346</a>.)</li>
<li>The <a class="reference internal" href="../c-api/sys.html#c.Py_FatalError" title="Py_FatalError"><code class="xref c c-func docutils literal notranslate">Py_FatalError()</code></a> function is replaced with a macro which logs
automatically the name of the current function, unless the
<code class="docutils literal notranslate">Py_LIMITED_API</code> macro is defined.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39882">bpo-39882</a>.)</li>
<li>The vectorcall protocol now requires that the caller passes only strings as
keyword names. (See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37540">bpo-37540</a> for more information.)</li>
<li>Implementation details of a number of macros and functions are now hidden:
<ul class="simple">
<li><a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_IS_GC" title="PyObject_IS_GC"><code class="xref c c-func docutils literal notranslate">PyObject_IS_GC()</code></a> macro was converted to a function.</li>
<li>The <code class="xref c c-func docutils literal notranslate">PyObject_NEW()</code> macro becomes an alias to the
<a class="reference internal" href="../c-api/allocation.html#c.PyObject_New" title="PyObject_New"><code class="xref c c-func docutils literal notranslate">PyObject_New()</code></a> macro, and the <code class="xref c c-func docutils literal notranslate">PyObject_NEW_VAR()</code> macro
becomes an alias to the <a class="reference internal" href="../c-api/allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_NewVar()</code></a> macro. They no longer
access directly the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_basicsize" title="PyTypeObject.tp_basicsize"><code class="xref c c-member docutils literal notranslate">PyTypeObject.tp_basicsize</code></a> member.</li>
<li><code class="xref c c-func docutils literal notranslate">PyObject_GET_WEAKREFS_LISTPTR()</code> macro was converted to a function:
the macro accessed directly the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_weaklistoffset" title="PyTypeObject.tp_weaklistoffset"><code class="xref c c-member docutils literal notranslate">PyTypeObject.tp_weaklistoffset</code></a>
member.</li>
<li><a class="reference internal" href="../c-api/buffer.html#c.PyObject_CheckBuffer" title="PyObject_CheckBuffer"><code class="xref c c-func docutils literal notranslate">PyObject_CheckBuffer()</code></a> macro was converted to a function: the macro
accessed directly the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_as_buffer" title="PyTypeObject.tp_as_buffer"><code class="xref c c-member docutils literal notranslate">PyTypeObject.tp_as_buffer</code></a> member.</li>
<li><a class="reference internal" href="../c-api/number.html#c.PyIndex_Check" title="PyIndex_Check"><code class="xref c c-func docutils literal notranslate">PyIndex_Check()</code></a> is now always declared as an opaque function to hide
implementation details: removed the <code class="docutils literal notranslate">PyIndex_Check()</code> macro. The macro accessed
directly the <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_as_number" title="PyTypeObject.tp_as_number"><code class="xref c c-member docutils literal notranslate">PyTypeObject.tp_as_number</code></a> member.</li>
</ul>
(See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40170">bpo-40170</a> for more details.)
</li>
</ul>
</section>
<section id="id3">
<h3>Removed<a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h3>
<ul>
<li>Excluded <code class="docutils literal notranslate">PyFPE_START_PROTECT()</code> and <code class="docutils literal notranslate">PyFPE_END_PROTECT()</code> macros of
<code class="docutils literal notranslate">pyfpe.h</code> from the limited C API.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38835">bpo-38835</a>.)</li>
<li>The <code class="docutils literal notranslate">tp_print</code> slot of <a class="reference internal" href="../c-api/typeobj.html#type-structs">PyTypeObject</a> has been removed.
It was used for printing objects to files in Python 2.7 and before. Since
Python 3.0, it has been ignored and unused.
(Contributed by Jeroen Demeyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36974">bpo-36974</a>.)</li>
<li>Changes in the limited C API (if <code class="docutils literal notranslate">Py_LIMITED_API</code> macro is defined):
<ul class="simple">
<li>Excluded the following functions from the limited C API:
<ul>
<li><code class="docutils literal notranslate">PyThreadState_DeleteCurrent()</code>
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37878">bpo-37878</a>.)</li>
<li><code class="docutils literal notranslate">_Py_CheckRecursionLimit</code></li>
<li><code class="docutils literal notranslate">_Py_NewReference()</code></li>
<li><code class="docutils literal notranslate">_Py_ForgetReference()</code></li>
<li><code class="docutils literal notranslate">_PyTraceMalloc_NewReference()</code></li>
<li><code class="docutils literal notranslate">_Py_GetRefTotal()</code></li>
<li>The trashcan mechanism which never worked in the limited C API.</li>
<li><code class="docutils literal notranslate">PyTrash_UNWIND_LEVEL</code></li>
<li><code class="docutils literal notranslate">Py_TRASHCAN_BEGIN_CONDITION</code></li>
<li><code class="docutils literal notranslate">Py_TRASHCAN_BEGIN</code></li>
<li><code class="docutils literal notranslate">Py_TRASHCAN_END</code></li>
<li><code class="docutils literal notranslate">Py_TRASHCAN_SAFE_BEGIN</code></li>
<li><code class="docutils literal notranslate">Py_TRASHCAN_SAFE_END</code></li>
</ul>
</li>
<li>Moved following functions and definitions to the internal C API:
<ul>
<li><code class="docutils literal notranslate">_PyDebug_PrintTotalRefs()</code></li>
<li><code class="docutils literal notranslate">_Py_PrintReferences()</code></li>
<li><code class="docutils literal notranslate">_Py_PrintReferenceAddresses()</code></li>
<li><code class="docutils literal notranslate">_Py_tracemalloc_config</code></li>
<li><code class="docutils literal notranslate">_Py_AddToAllObjects()</code> (specific to <code class="docutils literal notranslate">Py_TRACE_REFS</code> build)</li>
</ul>
</li>
</ul>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38644">bpo-38644</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39542">bpo-39542</a>.)
</li>
<li>Removed <code class="docutils literal notranslate">_PyRuntime.getframe</code> hook and removed <code class="docutils literal notranslate">_PyThreadState_GetFrame</code>
macro which was an alias to <code class="docutils literal notranslate">_PyRuntime.getframe</code>. They were only exposed
by the internal C API. Removed also <code class="docutils literal notranslate">PyThreadFrameGetter</code> type.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39946">bpo-39946</a>.)</li>
<li>Removed the following functions from the C API. Call <a class="reference internal" href="../c-api/gcsupport.html#c.PyGC_Collect" title="PyGC_Collect"><code class="xref c c-func docutils literal notranslate">PyGC_Collect()</code></a>
explicitly to clear all free lists.
(Contributed by Inada Naoki and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37340">bpo-37340</a>,
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38896">bpo-38896</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=40428">bpo-40428</a>.)
<ul class="simple">
<li><code class="docutils literal notranslate">PyAsyncGen_ClearFreeLists()</code></li>
<li><code class="docutils literal notranslate">PyContext_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyDict_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyFloat_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyFrame_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyList_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyMethod_ClearFreeList()</code> and <code class="docutils literal notranslate">PyCFunction_ClearFreeList()</code>:
the free lists of bound method objects have been removed.</li>
<li><code class="docutils literal notranslate">PySet_ClearFreeList()</code>: the set free list has been removed
in Python 3.4.</li>
<li><code class="docutils literal notranslate">PyTuple_ClearFreeList()</code></li>
<li><code class="docutils literal notranslate">PyUnicode_ClearFreeList()</code>: the Unicode free list has been removed in
Python 3.3.</li>
</ul>
</li>
<li>Removed <code class="docutils literal notranslate">_PyUnicode_ClearStaticStrings()</code> function.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39465">bpo-39465</a>.)</li>
<li>Removed <code class="docutils literal notranslate">Py_UNICODE_MATCH</code>. It has been deprecated by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0393">PEP 393</a>, and
broken since Python 3.3. The <a class="reference internal" href="../c-api/unicode.html#c.PyUnicode_Tailmatch" title="PyUnicode_Tailmatch"><code class="xref c c-func docutils literal notranslate">PyUnicode_Tailmatch()</code></a> function can be
used instead.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36346">bpo-36346</a>.)</li>
<li>Cleaned header files of interfaces defined but with no implementation.
The public API symbols being removed are:
<code class="docutils literal notranslate">_PyBytes_InsertThousandsGroupingLocale</code>,
<code class="docutils literal notranslate">_PyBytes_InsertThousandsGrouping</code>, <code class="docutils literal notranslate">_Py_InitializeFromArgs</code>,
<code class="docutils literal notranslate">_Py_InitializeFromWideArgs</code>, <code class="docutils literal notranslate">_PyFloat_Repr</code>, <code class="docutils literal notranslate">_PyFloat_Digits</code>,
<code class="docutils literal notranslate">_PyFloat_DigitsInit</code>, <code class="docutils literal notranslate">PyFrame_ExtendStack</code>, <code class="docutils literal notranslate">_PyAIterWrapper_Type</code>,
<code class="docutils literal notranslate">PyNullImporter_Type</code>, <code class="docutils literal notranslate">PyCmpWrapper_Type</code>, <code class="docutils literal notranslate">PySortWrapper_Type</code>,
<code class="docutils literal notranslate">PyNoArgsFunction</code>.
(Contributed by Pablo Galindo Salgado in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=39372">bpo-39372</a>.)</li>
</ul>
</section>
</section>
<section id="notable-changes-in-python-3-9-1">
<h2>Notable changes in Python 3.9.1<a class="headerlink" href="#notable-changes-in-python-3-9-1" title="Permalink to this headline">¶</a></h2>
<section id="id4">
<h3>typing<a class="headerlink" href="#id4" title="Permalink to this headline">¶</a></h3>
The behavior of <a class="reference internal" href="../library/typing.html#typing.Literal" title="typing.Literal"><code class="xref py py-class docutils literal notranslate">typing.Literal</code></a> was changed to conform with <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0586">PEP 586</a>
and to match the behavior of static type checkers specified in the PEP.
<ol class="arabic">
<li><code class="docutils literal notranslate">Literal</code> now de-duplicates parameters.</li>
<li>Equality comparisons between <code class="docutils literal notranslate">Literal</code> objects are now order independent.</li>
<li><code class="docutils literal notranslate">Literal</code> comparisons now respect types. For example,
<code class="docutils literal notranslate">Literal[0] == Literal[False]</code> previously evaluated to <code class="docutils literal notranslate">True</code>. It is
now <code class="docutils literal notranslate">False</code>. To support this change, the internally used type cache now
supports differentiating types.</li>
<li><code class="docutils literal notranslate">Literal</code> objects will now raise a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> exception during
equality comparisons if any of their parameters are not <a class="reference internal" href="../glossary.html#term-hashable">hashable</a>.
Note that declaring <code class="docutils literal notranslate">Literal</code> with mutable parameters will not throw
an error:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from typing import Literal
&gt;&gt;&gt; Literal[{0}]
&gt;&gt;&gt; Literal[{0}] == Literal[{False}]
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
TypeError: unhashable type: &#39;set&#39;
</pre></div>
</div>
</li>
</ol>
(Contributed by Yurii Karabas in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=42345">bpo-42345</a>.)
</section>
<section id="macos-11-0-big-sur-and-apple-silicon-mac-support">
<h3>macOS 11.0 (Big Sur) and Apple Silicon Mac support<a class="headerlink" href="#macos-11-0-big-sur-and-apple-silicon-mac-support" title="Permalink to this headline">¶</a></h3>
As of 3.9.1, Python now fully supports building and running on macOS 11.0
(Big Sur) and on Apple Silicon Macs (based on the <code class="docutils literal notranslate">ARM64</code> architecture).
A new universal build variant, <code class="docutils literal notranslate">universal2</code>, is now available to natively
support both <code class="docutils literal notranslate">ARM64</code> and <code class="docutils literal notranslate">Intel 64</code> in one set of executables. Binaries
can also now be built on current versions of macOS to be deployed on a range
of older macOS versions (tested to 10.9) while making some newer OS
functions and options conditionally available based on the operating system
version in use at runtime (“weaklinking”).
(Contributed by Ronald Oussoren and Lawrence D’Anna in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=41100">bpo-41100</a>.)
</section>
</section>
<section id="notable-changes-in-python-3-9-2">
<h2>Notable changes in Python 3.9.2<a class="headerlink" href="#notable-changes-in-python-3-9-2" title="Permalink to this headline">¶</a></h2>
<section id="collections-abc">
<h3>collections.abc<a class="headerlink" href="#collections-abc" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/collections.abc.html#collections.abc.Callable" title="collections.abc.Callable"><code class="xref py py-class docutils literal notranslate">collections.abc.Callable</code></a> generic now flattens type parameters, similar
to what <a class="reference internal" href="../library/typing.html#typing.Callable" title="typing.Callable"><code class="xref py py-data docutils literal notranslate">typing.Callable</code></a> currently does. This means that
<code class="docutils literal notranslate">collections.abc.Callable[[int, str], str]</code> will have <code class="docutils literal notranslate">__args__</code> of
<code class="docutils literal notranslate">(int, str, str)</code>; previously this was <code class="docutils literal notranslate">([int, str], str)</code>. To allow this
change, <a class="reference internal" href="../library/types.html#types.GenericAlias" title="types.GenericAlias"><code class="xref py py-class docutils literal notranslate">types.GenericAlias</code></a> can now be subclassed, and a subclass will
be returned when subscripting the <a class="reference internal" href="../library/collections.abc.html#collections.abc.Callable" title="collections.abc.Callable"><code class="xref py py-class docutils literal notranslate">collections.abc.Callable</code></a> type.
Code which accesses the arguments via <a class="reference internal" href="../library/typing.html#typing.get_args" title="typing.get_args"><code class="xref py py-func docutils literal notranslate">typing.get_args()</code></a> or <code class="docutils literal notranslate">__args__</code>
need to account for this change. A <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> may be emitted for
invalid forms of parameterizing <a class="reference internal" href="../library/collections.abc.html#collections.abc.Callable" title="collections.abc.Callable"><code class="xref py py-class docutils literal notranslate">collections.abc.Callable</code></a> which may have
passed silently in Python 3.9.1. This <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> will
become a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> in Python 3.10.
(Contributed by Ken Jin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=42195">bpo-42195</a>.)
</section>
<section id="urllib-parse">
<h3>urllib.parse<a class="headerlink" href="#urllib-parse" title="Permalink to this headline">¶</a></h3>
Earlier Python versions allowed using both <code class="docutils literal notranslate">;</code> and <code class="docutils literal notranslate">&amp;</code> as
query parameter separators in <a class="reference internal" href="../library/urllib.parse.html#urllib.parse.parse_qs" title="urllib.parse.parse_qs"><code class="xref py py-func docutils literal notranslate">urllib.parse.parse_qs()</code></a> and
<a class="reference internal" href="../library/urllib.parse.html#urllib.parse.parse_qsl" title="urllib.parse.parse_qsl"><code class="xref py py-func docutils literal notranslate">urllib.parse.parse_qsl()</code></a>. Due to security concerns, and to conform with
newer W3C recommendations, this has been changed to allow only a single
separator key, with <code class="docutils literal notranslate">&amp;</code> as the default. This change also affects
<a class="reference internal" href="../library/cgi.html#cgi.parse" title="cgi.parse"><code class="xref py py-func docutils literal notranslate">cgi.parse()</code></a> and <a class="reference internal" href="../library/cgi.html#cgi.parse_multipart" title="cgi.parse_multipart"><code class="xref py py-func docutils literal notranslate">cgi.parse_multipart()</code></a> as they use the affected
functions internally. For more details, please see their respective
documentation.
(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=42967">bpo-42967</a>.)
</section>
</section>
</section>

<div class="clearer"></div>
 </div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">What’s New In Python 3.9</a><ul>
<li><a class="reference internal" href="#summary-release-highlights">Summary – Release highlights</a></li>
<li><a class="reference internal" href="#you-should-check-for-deprecationwarning-in-your-code">You should check for DeprecationWarning in your code</a></li>
<li><a class="reference internal" href="#new-features">New Features</a><ul>
<li><a class="reference internal" href="#dictionary-merge-update-operators">Dictionary Merge &amp; Update Operators</a></li>
<li><a class="reference internal" href="#new-string-methods-to-remove-prefixes-and-suffixes">New String Methods to Remove Prefixes and Suffixes</a></li>
<li><a class="reference internal" href="#type-hinting-generics-in-standard-collections">Type Hinting Generics in Standard Collections</a></li>
<li><a class="reference internal" href="#new-parser">New Parser</a></li>
</ul>
</li>
<li><a class="reference internal" href="#other-language-changes">Other Language Changes</a></li>
<li><a class="reference internal" href="#new-modules">New Modules</a><ul>
<li><a class="reference internal" href="#zoneinfo">zoneinfo</a></li>
<li><a class="reference internal" href="#graphlib">graphlib</a></li>
</ul>
</li>
<li><a class="reference internal" href="#improved-modules">Improved Modules</a><ul>
<li><a class="reference internal" href="#ast">ast</a></li>
<li><a class="reference internal" href="#asyncio">asyncio</a></li>
<li><a class="reference internal" href="#compileall">compileall</a></li>
<li><a class="reference internal" href="#concurrent-futures">concurrent.futures</a></li>
<li><a class="reference internal" href="#curses">curses</a></li>
<li><a class="reference internal" href="#datetime">datetime</a></li>
<li><a class="reference internal" href="#distutils">distutils</a></li>
<li><a class="reference internal" href="#fcntl">fcntl</a></li>
<li><a class="reference internal" href="#ftplib">ftplib</a></li>
<li><a class="reference internal" href="#gc">gc</a></li>
<li><a class="reference internal" href="#hashlib">hashlib</a></li>
<li><a class="reference internal" href="#http">http</a></li>
<li><a class="reference internal" href="#idle-and-idlelib">IDLE and idlelib</a></li>
<li><a class="reference internal" href="#imaplib">imaplib</a></li>
<li><a class="reference internal" href="#importlib">importlib</a></li>
<li><a class="reference internal" href="#inspect">inspect</a></li>
<li><a class="reference internal" href="#ipaddress">ipaddress</a></li>
<li><a class="reference internal" href="#math">math</a></li>
<li><a class="reference internal" href="#multiprocessing">multiprocessing</a></li>
<li><a class="reference internal" href="#nntplib">nntplib</a></li>
<li><a class="reference internal" href="#os">os</a></li>
<li><a class="reference internal" href="#pathlib">pathlib</a></li>
<li><a class="reference internal" href="#pdb">pdb</a></li>
<li><a class="reference internal" href="#poplib">poplib</a></li>
<li><a class="reference internal" href="#pprint">pprint</a></li>
<li><a class="reference internal" href="#pydoc">pydoc</a></li>
<li><a class="reference internal" href="#random">random</a></li>
<li><a class="reference internal" href="#signal">signal</a></li>
<li><a class="reference internal" href="#smtplib">smtplib</a></li>
<li><a class="reference internal" href="#socket">socket</a></li>
<li><a class="reference internal" href="#time">time</a></li>
<li><a class="reference internal" href="#sys">sys</a></li>
<li><a class="reference internal" href="#tracemalloc">tracemalloc</a></li>
<li><a class="reference internal" href="#typing">typing</a></li>
<li><a class="reference internal" href="#unicodedata">unicodedata</a></li>
<li><a class="reference internal" href="#venv">venv</a></li>
<li><a class="reference internal" href="#xml">xml</a></li>
</ul>
</li>
<li><a class="reference internal" href="#optimizations">Optimizations</a></li>
<li><a class="reference internal" href="#deprecated">Deprecated</a></li>
<li><a class="reference internal" href="#removed">Removed</a></li>
<li><a class="reference internal" href="#porting-to-python-3-9">Porting to Python 3.9</a><ul>
<li><a class="reference internal" href="#changes-in-the-python-api">Changes in the Python API</a></li>
<li><a class="reference internal" href="#changes-in-the-c-api">Changes in the C API</a></li>
<li><a class="reference internal" href="#cpython-bytecode-changes">CPython bytecode changes</a></li>
</ul>
</li>
<li><a class="reference internal" href="#build-changes">Build Changes</a></li>
<li><a class="reference internal" href="#c-api-changes">C API Changes</a><ul>
<li><a class="reference internal" href="#id1">New Features</a></li>
<li><a class="reference internal" href="#id2">Porting to Python 3.9</a></li>
<li><a class="reference internal" href="#id3">Removed</a></li>
</ul>
</li>
<li><a class="reference internal" href="#notable-changes-in-python-3-9-1">Notable changes in Python 3.9.1</a><ul>
<li><a class="reference internal" href="#id4">typing</a></li>
<li><a class="reference internal" href="#macos-11-0-big-sur-and-apple-silicon-mac-support">macOS 11.0 (Big Sur) and Apple Silicon Mac support</a></li>
</ul>
</li>
<li><a class="reference internal" href="#notable-changes-in-python-3-9-2">Notable changes in Python 3.9.2</a><ul>
<li><a class="reference internal" href="#collections-abc">collections.abc</a></li>
<li><a class="reference internal" href="#urllib-parse">urllib.parse</a></li>
</ul>
</li>
</ul>
</li>
</ul>

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