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 <h3><a href="../contents.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">What’s New In Python 3.0</a><ul>
<li><a class="reference internal" href="#common-stumbling-blocks">Common Stumbling Blocks</a><ul>
<li><a class="reference internal" href="#print-is-a-function">Print Is A Function</a></li>
<li><a class="reference internal" href="#views-and-iterators-instead-of-lists">Views And Iterators Instead Of Lists</a></li>
<li><a class="reference internal" href="#ordering-comparisons">Ordering Comparisons</a></li>
<li><a class="reference internal" href="#integers">Integers</a></li>
<li><a class="reference internal" href="#text-vs-data-instead-of-unicode-vs-8-bit">Text Vs. Data Instead Of Unicode Vs. 8-bit</a></li>
</ul>
</li>
<li><a class="reference internal" href="#overview-of-syntax-changes">Overview Of Syntax Changes</a><ul>
<li><a class="reference internal" href="#new-syntax">New Syntax</a></li>
<li><a class="reference internal" href="#changed-syntax">Changed Syntax</a></li>
<li><a class="reference internal" href="#removed-syntax">Removed Syntax</a></li>
</ul>
</li>
<li><a class="reference internal" href="#changes-already-present-in-python-2-6">Changes Already Present In Python 2.6</a></li>
<li><a class="reference internal" href="#library-changes">Library Changes</a></li>
<li><a class="reference internal" href="#pep-3101-a-new-approach-to-string-formatting">PEP 3101: A New Approach To String Formatting</a></li>
<li><a class="reference internal" href="#changes-to-exceptions">Changes To Exceptions</a></li>
<li><a class="reference internal" href="#miscellaneous-other-changes">Miscellaneous Other Changes</a><ul>
<li><a class="reference internal" href="#operators-and-special-methods">Operators And Special Methods</a></li>
<li><a class="reference internal" href="#builtins">Builtins</a></li>
</ul>
</li>
<li><a class="reference internal" href="#build-and-c-api-changes">Build and C API Changes</a></li>
<li><a class="reference internal" href="#performance">Performance</a></li>
<li><a class="reference internal" href="#porting-to-python-3-0">Porting To Python 3.0</a></li>
</ul>
</li>
</ul>

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<li class="nav-item nav-item-1"><a href="index.html" accesskey="U">What’s New in Python</a> &#187;</li>
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 <section id="what-s-new-in-python-3-0">
<h1>What’s New In Python 3.0<a class="headerlink" href="#what-s-new-in-python-3-0" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd">Guido van Rossum
</dd>
</dl>
This article explains the new features in Python 3.0, compared to 2.6.
Python 3.0, also known as “Python 3000” or “Py3K”, is the first ever
intentionally backwards incompatible Python release. Python 3.0 was released on December 3, 2008.
There are more changes than in a typical release, and more that are important for all
Python users. Nevertheless, after digesting the changes, you’ll find
that Python really hasn’t changed all that much – by and large, we’re
mostly fixing well-known annoyances and warts, and removing a lot of
old cruft.
This article doesn’t attempt to provide a complete specification of
all new features, but instead tries to give a convenient overview.
For full details, you should refer to the documentation for Python
3.0, and/or the many PEPs referenced in the text. If you want to
understand the complete implementation and design rationale for a
particular feature, PEPs usually have more details than the regular
documentation; but note that PEPs usually are not kept up-to-date once
a feature has been fully implemented.
Due to time constraints this document is not as complete as it should
have been. As always for a new release, the <code class="docutils literal notranslate">Misc/NEWS</code> file in the
source distribution contains a wealth of detailed information about
every small thing that was changed.
<section id="common-stumbling-blocks">
<h2>Common Stumbling Blocks<a class="headerlink" href="#common-stumbling-blocks" title="Permalink to this headline">¶</a></h2>
This section lists those few changes that are most likely to trip you
up if you’re used to Python 2.5.
<section id="print-is-a-function">
<h3>Print Is A Function<a class="headerlink" href="#print-is-a-function" title="Permalink to this headline">¶</a></h3>
The <code class="docutils literal notranslate">print</code> statement has been replaced with a <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a>
function, with keyword arguments to replace most of the special syntax
of the old <code class="docutils literal notranslate">print</code> statement (<a class="pep reference external" href="https://www.python.org/dev/peps/pep-3105">PEP 3105</a>). Examples:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>Old: print &quot;The answer is&quot;, 2*2
New: print(&quot;The answer is&quot;, 2*2)

Old: print x, # Trailing comma suppresses newline
New: print(x, end=&quot; &quot;) # Appends a space instead of a newline

Old: print # Prints a newline
New: print() # You must call the function!

Old: print &gt;&gt;sys.stderr, &quot;fatal error&quot;
New: print(&quot;fatal error&quot;, file=sys.stderr)

Old: print (x, y) # prints repr((x, y))
New: print((x, y)) # Not the same as print(x, y)!
</pre></div>
</div>
You can also customize the separator between items, e.g.:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>print(&quot;There are &lt;&quot;, 2**32, &quot;&gt; possibilities!&quot;, sep=&quot;&quot;)
</pre></div>
</div>
which produces:
<div class="highlight-none notranslate"><div class="highlight"><pre>There are &lt;4294967296&gt; possibilities!
</pre></div>
</div>
Note:
<ul class="simple">
<li>The <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> function doesn’t support the “softspace” feature of
the old <code class="docutils literal notranslate">print</code> statement. For example, in Python 2.x,
<code class="docutils literal notranslate">print &quot;A\n&quot;, &quot;B&quot;</code> would write <code class="docutils literal notranslate">&quot;A\nB\n&quot;</code>; but in Python 3.0,
<code class="docutils literal notranslate">print(&quot;A\n&quot;, &quot;B&quot;)</code> writes <code class="docutils literal notranslate">&quot;A\n B\n&quot;</code>.</li>
<li>Initially, you’ll be finding yourself typing the old <code class="docutils literal notranslate">print x</code>
a lot in interactive mode. Time to retrain your fingers to type
<code class="docutils literal notranslate">print(x)</code> instead!</li>
<li>When using the <code class="docutils literal notranslate">2to3</code> source-to-source conversion tool, all
<code class="docutils literal notranslate">print</code> statements are automatically converted to
<a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate">print()</code></a> function calls, so this is mostly a non-issue for
larger projects.</li>
</ul>
</section>
<section id="views-and-iterators-instead-of-lists">
<h3>Views And Iterators Instead Of Lists<a class="headerlink" href="#views-and-iterators-instead-of-lists" title="Permalink to this headline">¶</a></h3>
Some well-known APIs no longer return lists:
<ul>
<li><a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> methods <a class="reference internal" href="../library/stdtypes.html#dict.keys" title="dict.keys"><code class="xref py py-meth docutils literal notranslate">dict.keys()</code></a>, <a class="reference internal" href="../library/stdtypes.html#dict.items" title="dict.items"><code class="xref py py-meth docutils literal notranslate">dict.items()</code></a> and
<a class="reference internal" href="../library/stdtypes.html#dict.values" title="dict.values"><code class="xref py py-meth docutils literal notranslate">dict.values()</code></a> return “views” instead of lists. For example,
this no longer works: <code class="docutils literal notranslate">k = d.keys(); k.sort()</code>. Use <code class="docutils literal notranslate">k =
sorted(d)</code> instead (this works in Python 2.5 too and is just
as efficient).</li>
<li>Also, the <code class="xref py py-meth docutils literal notranslate">dict.iterkeys()</code>, <code class="xref py py-meth docutils literal notranslate">dict.iteritems()</code> and
<code class="xref py py-meth docutils literal notranslate">dict.itervalues()</code> methods are no longer supported.</li>
<li><a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a> and <a class="reference internal" href="../library/functions.html#filter" title="filter"><code class="xref py py-func docutils literal notranslate">filter()</code></a> return iterators. If you really need
a list and the input sequences are all of equal length, a quick
fix is to wrap <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a> in <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-func docutils literal notranslate">list()</code></a>, e.g. <code class="docutils literal notranslate">list(map(...))</code>,
but a better fix is
often to use a list comprehension (especially when the original code
uses <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate">lambda</code></a>), or rewriting the code so it doesn’t need a
list at all. Particularly tricky is <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a> invoked for the
side effects of the function; the correct transformation is to use a
regular <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> loop (since creating a list would just be
wasteful).
If the input sequences are not of equal length, <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a> will
stop at the termination of the shortest of the sequences. For full
compatibility with <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map()</code></a> from Python 2.x, also wrap the sequences in
<a class="reference internal" href="../library/itertools.html#itertools.zip_longest" title="itertools.zip_longest"><code class="xref py py-func docutils literal notranslate">itertools.zip_longest()</code></a>, e.g. <code class="docutils literal notranslate">map(func, *sequences)</code> becomes
<code class="docutils literal notranslate">list(map(func, itertools.zip_longest(*sequences)))</code>.
</li>
<li><a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate">range()</code></a> now behaves like <code class="xref py py-func docutils literal notranslate">xrange()</code> used to behave, except
it works with values of arbitrary size. The latter no longer
exists.</li>
<li><a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate">zip()</code></a> now returns an iterator.</li>
</ul>
</section>
<section id="ordering-comparisons">
<h3>Ordering Comparisons<a class="headerlink" href="#ordering-comparisons" title="Permalink to this headline">¶</a></h3>
Python 3.0 has simplified the rules for ordering comparisons:
<ul class="simple">
<li>The ordering comparison operators (<code class="docutils literal notranslate">&lt;</code>, <code class="docutils literal notranslate">&lt;=</code>, <code class="docutils literal notranslate">&gt;=</code>, <code class="docutils literal notranslate">&gt;</code>)
raise a TypeError exception when the operands don’t have a
meaningful natural ordering. Thus, expressions like <code class="docutils literal notranslate">1 &lt; ''</code>, <code class="docutils literal notranslate">0
&gt; None</code> or <code class="docutils literal notranslate">len &lt;= len</code> are no longer valid, and e.g. <code class="docutils literal notranslate">None &lt;
None</code> raises <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> instead of returning
<code class="docutils literal notranslate">False</code>. A corollary is that sorting a heterogeneous list
no longer makes sense – all the elements must be comparable to each
other. Note that this does not apply to the <code class="docutils literal notranslate">==</code> and <code class="docutils literal notranslate">!=</code>
operators: objects of different incomparable types always compare
unequal to each other.</li>
<li><code class="xref py py-meth docutils literal notranslate">builtin.sorted()</code> and <a class="reference internal" href="../library/stdtypes.html#list.sort" title="list.sort"><code class="xref py py-meth docutils literal notranslate">list.sort()</code></a> no longer accept the
cmp argument providing a comparison function. Use the key
argument instead. N.B. the key and reverse arguments are now
“keyword-only”.</li>
<li>The <code class="xref py py-func docutils literal notranslate">cmp()</code> function should be treated as gone, and the <code class="xref py py-meth docutils literal notranslate">__cmp__()</code>
special method is no longer supported. Use <code class="xref py py-meth docutils literal notranslate">__lt__()</code> for sorting,
<code class="xref py py-meth docutils literal notranslate">__eq__()</code> with <code class="xref py py-meth docutils literal notranslate">__hash__()</code>, and other rich comparisons as needed.
(If you really need the <code class="xref py py-func docutils literal notranslate">cmp()</code> functionality, you could use the
expression <code class="docutils literal notranslate">(a &gt; b) - (a &lt; b)</code> as the equivalent for <code class="docutils literal notranslate">cmp(a, b)</code>.)</li>
</ul>
</section>
<section id="integers">
<h3>Integers<a class="headerlink" href="#integers" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0237">PEP 237</a>: Essentially, <code class="xref py py-class docutils literal notranslate">long</code> renamed to <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>.
That is, there is only one built-in integral type, named
<a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>; but it behaves mostly like the old <code class="xref py py-class docutils literal notranslate">long</code> type.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0238">PEP 238</a>: An expression like <code class="docutils literal notranslate">1/2</code> returns a float. Use
<code class="docutils literal notranslate">1//2</code> to get the truncating behavior. (The latter syntax has
existed for years, at least since Python 2.2.)</li>
<li>The <code class="xref py py-data docutils literal notranslate">sys.maxint</code> constant was removed, since there is no
longer a limit to the value of integers. However, <a class="reference internal" href="../library/sys.html#sys.maxsize" title="sys.maxsize"><code class="xref py py-data docutils literal notranslate">sys.maxsize</code></a>
can be used as an integer larger than any practical list or string
index. It conforms to the implementation’s “natural” integer size
and is typically the same as <code class="xref py py-data docutils literal notranslate">sys.maxint</code> in previous releases
on the same platform (assuming the same build options).</li>
<li>The <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> of a long integer doesn’t include the trailing <code class="docutils literal notranslate">L</code>
anymore, so code that unconditionally strips that character will
chop off the last digit instead. (Use <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate">str()</code></a> instead.)</li>
<li>Octal literals are no longer of the form <code class="docutils literal notranslate">0720</code>; use <code class="docutils literal notranslate">0o720</code>
instead.</li>
</ul>
</section>
<section id="text-vs-data-instead-of-unicode-vs-8-bit">
<h3>Text Vs. Data Instead Of Unicode Vs. 8-bit<a class="headerlink" href="#text-vs-data-instead-of-unicode-vs-8-bit" title="Permalink to this headline">¶</a></h3>
Everything you thought you knew about binary data and Unicode has
changed.
<ul class="simple">
<li>Python 3.0 uses the concepts of text and (binary) data instead
of Unicode strings and 8-bit strings. All text is Unicode; however
encoded Unicode is represented as binary data. The type used to
hold text is <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, the type used to hold data is
<a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>. The biggest difference with the 2.x situation is
that any attempt to mix text and data in Python 3.0 raises
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>, whereas if you were to mix Unicode and 8-bit
strings in Python 2.x, it would work if the 8-bit string happened to
contain only 7-bit (ASCII) bytes, but you would get
<a class="reference internal" href="../library/exceptions.html#UnicodeDecodeError" title="UnicodeDecodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeDecodeError</code></a> if it contained non-ASCII values. This
value-specific behavior has caused numerous sad faces over the
years.</li>
<li>As a consequence of this change in philosophy, pretty much all code
that uses Unicode, encodings or binary data most likely has to
change. The change is for the better, as in the 2.x world there
were numerous bugs having to do with mixing encoded and unencoded
text. To be prepared in Python 2.x, start using <code class="xref py py-class docutils literal notranslate">unicode</code>
for all unencoded text, and <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> for binary or encoded data
only. Then the <code class="docutils literal notranslate">2to3</code> tool will do most of the work for you.</li>
<li>You can no longer use <code class="docutils literal notranslate">u&quot;...&quot;</code> literals for Unicode text.
However, you must use <code class="docutils literal notranslate">b&quot;...&quot;</code> literals for binary data.</li>
<li>As the <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> and <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> types cannot be mixed, you
must always explicitly convert between them. Use <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>
to go from <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> to <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#bytes.decode" title="bytes.decode"><code class="xref py py-meth docutils literal notranslate">bytes.decode()</code></a>
to go from <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> to <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>. You can also use
<code class="docutils literal notranslate">bytes(s, encoding=...)</code> and <code class="docutils literal notranslate">str(b, encoding=...)</code>,
respectively.</li>
<li>Like <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, the <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> type is immutable. There is a
separate mutable type to hold buffered binary data,
<a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a>. Nearly all APIs that accept <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> also
accept <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a>. The mutable API is based on
<code class="xref py py-class docutils literal notranslate">collections.MutableSequence</code>.</li>
<li>All backslashes in raw string literals are interpreted literally.
This means that <code class="docutils literal notranslate">'\U'</code> and <code class="docutils literal notranslate">'\u'</code> escapes in raw strings are not
treated specially. For example, <code class="docutils literal notranslate">r'\u20ac'</code> is a string of 6
characters in Python 3.0, whereas in 2.6, <code class="docutils literal notranslate">ur'\u20ac'</code> was the
single “euro” character. (Of course, this change only affects raw
string literals; the euro character is <code class="docutils literal notranslate">'\u20ac'</code> in Python 3.0.)</li>
<li>The built-in <code class="xref py py-class docutils literal notranslate">basestring</code> abstract type was removed. Use
<a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> instead. The <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> and <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> types
don’t have functionality enough in common to warrant a shared base
class. The <code class="docutils literal notranslate">2to3</code> tool (see below) replaces every occurrence of
<code class="xref py py-class docutils literal notranslate">basestring</code> with <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>.</li>
<li>Files opened as text files (still the default mode for <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a>)
always use an encoding to map between strings (in memory) and bytes
(on disk). Binary files (opened with a <code class="docutils literal notranslate">b</code> in the mode argument)
always use bytes in memory. This means that if a file is opened
using an incorrect mode or encoding, I/O will likely fail loudly,
instead of silently producing incorrect data. It also means that
even Unix users will have to specify the correct mode (text or
binary) when opening a file. There is a platform-dependent default
encoding, which on Unixy platforms can be set with the <code class="docutils literal notranslate">LANG</code>
environment variable (and sometimes also with some other
platform-specific locale-related environment variables). In many
cases, but not all, the system default is UTF-8; you should never
count on this default. Any application reading or writing more than
pure ASCII text should probably have a way to override the encoding.
There is no longer any need for using the encoding-aware streams
in the <a class="reference internal" href="../library/codecs.html#module-codecs" title="codecs: Encode and decode data and streams."><code class="xref py py-mod docutils literal notranslate">codecs</code></a> module.</li>
<li>The initial values of <a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate">sys.stdin</code></a>, <a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a> and
<a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate">sys.stderr</code></a> are now unicode-only text files (i.e., they are
instances of <a class="reference internal" href="../library/io.html#io.TextIOBase" title="io.TextIOBase"><code class="xref py py-class docutils literal notranslate">io.TextIOBase</code></a>). To read and write bytes data
with these streams, you need to use their <a class="reference internal" href="../library/io.html#io.TextIOBase.buffer" title="io.TextIOBase.buffer"><code class="xref py py-data docutils literal notranslate">io.TextIOBase.buffer</code></a>
attribute.</li>
<li>Filenames are passed to and returned from APIs as (Unicode) strings.
This can present platform-specific problems because on some
platforms filenames are arbitrary byte strings. (On the other hand,
on Windows filenames are natively stored as Unicode.) As a
work-around, most APIs (e.g. <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a> and many functions in the
<a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate">os</code></a> module) that take filenames accept <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> objects
as well as strings, and a few APIs have a way to ask for a
<a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> return value. Thus, <a class="reference internal" href="../library/os.html#os.listdir" title="os.listdir"><code class="xref py py-func docutils literal notranslate">os.listdir()</code></a> returns a
list of <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> instances if the argument is a <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>
instance, and <a class="reference internal" href="../library/os.html#os.getcwdb" title="os.getcwdb"><code class="xref py py-func docutils literal notranslate">os.getcwdb()</code></a> returns the current working
directory as a <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> instance. Note that when
<a class="reference internal" href="../library/os.html#os.listdir" title="os.listdir"><code class="xref py py-func docutils literal notranslate">os.listdir()</code></a> returns a list of strings, filenames that
cannot be decoded properly are omitted rather than raising
<a class="reference internal" href="../library/exceptions.html#UnicodeError" title="UnicodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeError</code></a>.</li>
<li>Some system APIs like <a class="reference internal" href="../library/os.html#os.environ" title="os.environ"><code class="xref py py-data docutils literal notranslate">os.environ</code></a> and <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal notranslate">sys.argv</code></a> can
also present problems when the bytes made available by the system is
not interpretable using the default encoding. Setting the <code class="docutils literal notranslate">LANG</code>
variable and rerunning the program is probably the best approach.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3138">PEP 3138</a>: The <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> of a string no longer escapes
non-ASCII characters. It still escapes control characters and code
points with non-printable status in the Unicode standard, however.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3120">PEP 3120</a>: The default source encoding is now UTF-8.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3131">PEP 3131</a>: Non-ASCII letters are now allowed in identifiers.
(However, the standard library remains ASCII-only with the exception
of contributor names in comments.)</li>
<li>The <code class="xref py py-mod docutils literal notranslate">StringIO</code> and <code class="xref py py-mod docutils literal notranslate">cStringIO</code> modules are gone. Instead,
import the <a class="reference internal" href="../library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal notranslate">io</code></a> module and use <a class="reference internal" href="../library/io.html#io.StringIO" title="io.StringIO"><code class="xref py py-class docutils literal notranslate">io.StringIO</code></a> or
<a class="reference internal" href="../library/io.html#io.BytesIO" title="io.BytesIO"><code class="xref py py-class docutils literal notranslate">io.BytesIO</code></a> for text and data respectively.</li>
<li>See also the <a class="reference internal" href="../howto/unicode.html#unicode-howto">Unicode HOWTO</a>, which was updated for Python 3.0.</li>
</ul>
</section>
</section>
<section id="overview-of-syntax-changes">
<h2>Overview Of Syntax Changes<a class="headerlink" href="#overview-of-syntax-changes" title="Permalink to this headline">¶</a></h2>
This section gives a brief overview of every syntactic change in
Python 3.0.
<section id="new-syntax">
<h3>New Syntax<a class="headerlink" href="#new-syntax" title="Permalink to this headline">¶</a></h3>
<ul>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3107">PEP 3107</a>: Function argument and return value annotations. This
provides a standardized way of annotating a function’s parameters
and return value. There are no semantics attached to such
annotations except that they can be introspected at runtime using
the <code class="xref py py-attr docutils literal notranslate">__annotations__</code> attribute. The intent is to encourage
experimentation through metaclasses, decorators or frameworks.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3102">PEP 3102</a>: Keyword-only arguments. Named parameters occurring
after <code class="docutils literal notranslate">*args</code> in the parameter list must be specified using
keyword syntax in the call. You can also use a bare <code class="docutils literal notranslate">*</code> in the
parameter list to indicate that you don’t accept a variable-length
argument list, but you do have keyword-only arguments.</li>
<li>Keyword arguments are allowed after the list of base classes in a
class definition. This is used by the new convention for specifying
a metaclass (see next section), but can be used for other purposes
as well, as long as the metaclass supports it.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3104">PEP 3104</a>: <a class="reference internal" href="../reference/simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement. Using <code class="docutils literal notranslate">nonlocal x</code>
you can now assign directly to a variable in an outer (but
non-global) scope. <code class="xref std std-keyword docutils literal notranslate">nonlocal</code> is a new reserved word.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3132">PEP 3132</a>: Extended Iterable Unpacking. You can now write things
like <code class="docutils literal notranslate">a, b, *rest = some_sequence</code>. And even <code class="docutils literal notranslate">*rest, a =
stuff</code>. The <code class="docutils literal notranslate">rest</code> object is always a (possibly empty) list; the
right-hand side may be any iterable. Example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>(a, *rest, b) = range(5)
</pre></div>
</div>
This sets a to <code class="docutils literal notranslate">0</code>, b to <code class="docutils literal notranslate">4</code>, and rest to <code class="docutils literal notranslate">[1, 2, 3]</code>.
</li>
<li>Dictionary comprehensions: <code class="docutils literal notranslate">{k: v for k, v in stuff}</code> means the
same thing as <code class="docutils literal notranslate">dict(stuff)</code> but is more flexible. (This is
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0274">PEP 274</a> vindicated. :-)</li>
<li>Set literals, e.g. <code class="docutils literal notranslate">{1, 2}</code>. Note that <code class="docutils literal notranslate">{}</code> is an empty
dictionary; use <code class="docutils literal notranslate">set()</code> for an empty set. Set comprehensions are
also supported; e.g., <code class="docutils literal notranslate">{x for x in stuff}</code> means the same thing as
<code class="docutils literal notranslate">set(stuff)</code> but is more flexible.</li>
<li>New octal literals, e.g. <code class="docutils literal notranslate">0o720</code> (already in 2.6). The old octal
literals (<code class="docutils literal notranslate">0720</code>) are gone.</li>
<li>New binary literals, e.g. <code class="docutils literal notranslate">0b1010</code> (already in 2.6), and
there is a new corresponding built-in function, <a class="reference internal" href="../library/functions.html#bin" title="bin"><code class="xref py py-func docutils literal notranslate">bin()</code></a>.</li>
<li>Bytes literals are introduced with a leading <code class="docutils literal notranslate">b</code> or <code class="docutils literal notranslate">B</code>, and
there is a new corresponding built-in function, <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-func docutils literal notranslate">bytes()</code></a>.</li>
</ul>
</section>
<section id="changed-syntax">
<h3>Changed Syntax<a class="headerlink" href="#changed-syntax" title="Permalink to this headline">¶</a></h3>
<ul>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3109">PEP 3109</a> and <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3134">PEP 3134</a>: new <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement syntax:
<code class="samp docutils literal notranslate">raise [expr [from expr]]</code>. See below.</li>
<li><code class="xref std std-keyword docutils literal notranslate">as</code> and <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> are now reserved words. (Since
2.6, actually.)</li>
<li><code class="docutils literal notranslate">True</code>, <code class="docutils literal notranslate">False</code>, and <code class="docutils literal notranslate">None</code> are reserved words. (2.6 partially enforced
the restrictions on <code class="docutils literal notranslate">None</code> already.)</li>
<li>Change from <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> exc, var to
<code class="xref std std-keyword docutils literal notranslate">except</code> exc <code class="xref std std-keyword docutils literal notranslate">as</code> var. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3110">PEP 3110</a>.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3115">PEP 3115</a>: New Metaclass Syntax. Instead of:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class C:
 __metaclass__ = M
 ...
</pre></div>
</div>
you must now use:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class C(metaclass=M):
 ...
</pre></div>
</div>
The module-global <code class="xref py py-data docutils literal notranslate">__metaclass__</code> variable is no longer
supported. (It was a crutch to make it easier to default to
new-style classes without deriving every class from
<a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>.)
</li>
<li>List comprehensions no longer support the syntactic form
<code class="samp docutils literal notranslate">[... for var in item1, item2, ...]</code>. Use
<code class="samp docutils literal notranslate">[... for var in (item1, item2, ...)]</code> instead.
Also note that list comprehensions have different semantics: they
are closer to syntactic sugar for a generator expression inside a
<a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-func docutils literal notranslate">list()</code></a> constructor, and in particular the loop control
variables are no longer leaked into the surrounding scope.</li>
<li>The ellipsis (<code class="docutils literal notranslate">...</code>) can be used as an atomic expression
anywhere. (Previously it was only allowed in slices.) Also, it
must now be spelled as <code class="docutils literal notranslate">...</code>. (Previously it could also be
spelled as <code class="docutils literal notranslate">. . .</code>, by a mere accident of the grammar.)</li>
</ul>
</section>
<section id="removed-syntax">
<h3>Removed Syntax<a class="headerlink" href="#removed-syntax" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3113">PEP 3113</a>: Tuple parameter unpacking removed. You can no longer
write <code class="docutils literal notranslate">def foo(a, (b, c)): ...</code>.
Use <code class="docutils literal notranslate">def foo(a, b_c): b, c = b_c</code> instead.</li>
<li>Removed backticks (use <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate">repr()</code></a> instead).</li>
<li>Removed <code class="docutils literal notranslate">&lt;&gt;</code> (use <code class="docutils literal notranslate">!=</code> instead).</li>
<li>Removed keyword: <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate">exec()</code></a> is no longer a keyword; it remains as
a function. (Fortunately the function syntax was also accepted in
2.x.) Also note that <a class="reference internal" href="../library/functions.html#exec" title="exec"><code class="xref py py-func docutils literal notranslate">exec()</code></a> no longer takes a stream argument;
instead of <code class="docutils literal notranslate">exec(f)</code> you can use <code class="docutils literal notranslate">exec(f.read())</code>.</li>
<li>Integer literals no longer support a trailing <code class="docutils literal notranslate">l</code> or <code class="docutils literal notranslate">L</code>.</li>
<li>String literals no longer support a leading <code class="docutils literal notranslate">u</code> or <code class="docutils literal notranslate">U</code>.</li>
<li>The <a class="reference internal" href="../reference/simple_stmts.html#from"><code class="xref std std-keyword docutils literal notranslate">from</code></a> module <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> <code class="docutils literal notranslate">*</code> syntax is only
allowed at the module level, no longer inside functions.</li>
<li>The only acceptable syntax for relative imports is <code class="samp docutils literal notranslate">from .[module]
import name</code>. All <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> forms not starting with <code class="docutils literal notranslate">.</code> are
interpreted as absolute imports. (<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0328">PEP 328</a>)</li>
<li>Classic classes are gone.</li>
</ul>
</section>
</section>
<section id="changes-already-present-in-python-2-6">
<h2>Changes Already Present In Python 2.6<a class="headerlink" href="#changes-already-present-in-python-2-6" title="Permalink to this headline">¶</a></h2>
Since many users presumably make the jump straight from Python 2.5 to
Python 3.0, this section reminds the reader of new features that were
originally designed for Python 3.0 but that were back-ported to Python
2.6. The corresponding sections in <a class="reference internal" href="2.6.html#whats-new-in-2-6">What’s New in Python 2.6</a> should be
consulted for longer descriptions.
<ul class="simple">
<li><a class="reference internal" href="2.6.html#pep-0343">PEP 343: The ‘with’ statement</a>. The <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement is now a standard
feature and no longer needs to be imported from 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>.
Also check out <a class="reference internal" href="2.6.html#new-26-context-managers">Writing Context Managers</a> and
<a class="reference internal" href="2.6.html#new-module-contextlib">The contextlib module</a>.</li>
<li><a class="reference internal" href="2.6.html#pep-0366">PEP 366: Explicit Relative Imports From a Main Module</a>. This enhances the usefulness of the <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate">-m</code></a>
option when the referenced module lives in a package.</li>
<li><a class="reference internal" href="2.6.html#pep-0370">PEP 370: Per-user site-packages Directory</a>.</li>
<li><a class="reference internal" href="2.6.html#pep-0371">PEP 371: The multiprocessing Package</a>.</li>
<li><a class="reference internal" href="2.6.html#pep-3101">PEP 3101: Advanced String Formatting</a>. Note: the 2.6 description mentions the
<a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-meth docutils literal notranslate">format()</code></a> method for both 8-bit and Unicode strings. In 3.0,
only the <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a> type (text strings with Unicode support)
supports this method; the <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a> type does not. The plan is
to eventually make this the only API for string formatting, and to
start deprecating the <code class="docutils literal notranslate">%</code> operator in Python 3.1.</li>
<li><a class="reference internal" href="2.6.html#pep-3105">PEP 3105: print As a Function</a>. This is now a standard feature and no longer needs
to be imported from <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>. More details were given above.</li>
<li><a class="reference internal" href="2.6.html#pep-3110">PEP 3110: Exception-Handling Changes</a>. The <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> exc <code class="xref std std-keyword docutils literal notranslate">as</code> var
syntax is now standard and <code class="xref std std-keyword docutils literal notranslate">except</code> exc, var is no
longer supported. (Of course, the <code class="xref std std-keyword docutils literal notranslate">as</code> var part is still
optional.)</li>
<li><a class="reference internal" href="2.6.html#pep-3112">PEP 3112: Byte Literals</a>. The <code class="docutils literal notranslate">b&quot;...&quot;</code> string literal notation (and its
variants like <code class="docutils literal notranslate">b'...'</code>, <code class="docutils literal notranslate">b&quot;&quot;&quot;...&quot;&quot;&quot;</code>, and <code class="docutils literal notranslate">br&quot;...&quot;</code>) now
produces a literal of type <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>.</li>
<li><a class="reference internal" href="2.6.html#pep-3116">PEP 3116: New I/O Library</a>. The <a class="reference internal" href="../library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal notranslate">io</code></a> module is now the standard way of
doing file I/O. The built-in <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a> function is now an
alias for <a class="reference internal" href="../library/io.html#io.open" title="io.open"><code class="xref py py-func docutils literal notranslate">io.open()</code></a> and has additional keyword arguments
encoding, errors, newline and closefd. Also note that an
invalid mode argument now raises <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>, not
<a class="reference internal" href="../library/exceptions.html#IOError" title="IOError"><code class="xref py py-exc docutils literal notranslate">IOError</code></a>. The binary file object underlying a text file
object can be accessed as <code class="xref py py-attr docutils literal notranslate">f.buffer</code> (but beware that the
text object maintains a buffer of itself in order to speed up
the encoding and decoding operations).</li>
<li><a class="reference internal" href="2.6.html#pep-3118">PEP 3118: Revised Buffer Protocol</a>. The old builtin <code class="xref py py-func docutils literal notranslate">buffer()</code> is now really gone;
the new builtin <a class="reference internal" href="../library/stdtypes.html#memoryview" title="memoryview"><code class="xref py py-func docutils literal notranslate">memoryview()</code></a> provides (mostly) similar
functionality.</li>
<li><a class="reference internal" href="2.6.html#pep-3119">PEP 3119: Abstract Base Classes</a>. The <a class="reference internal" href="../library/abc.html#module-abc" title="abc: Abstract base classes according to :pep:`3119`."><code class="xref py py-mod docutils literal notranslate">abc</code></a> module and the ABCs defined 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 plays a somewhat more prominent role in
the language now, and built-in collection types like <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a>
and <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a> conform to the <code class="xref py py-class docutils literal notranslate">collections.MutableMapping</code>
and <code class="xref py py-class docutils literal notranslate">collections.MutableSequence</code> ABCs, respectively.</li>
<li><a class="reference internal" href="2.6.html#pep-3127">PEP 3127: Integer Literal Support and Syntax</a>. As mentioned above, the new octal literal
notation is the only one supported, and binary literals have been
added.</li>
<li><a class="reference internal" href="2.6.html#pep-3129">PEP 3129: Class Decorators</a>.</li>
<li><a class="reference internal" href="2.6.html#pep-3141">PEP 3141: A Type Hierarchy for Numbers</a>. The <a class="reference internal" href="../library/numbers.html#module-numbers" title="numbers: Numeric abstract base classes (Complex, Real, Integral, etc.)."><code class="xref py py-mod docutils literal notranslate">numbers</code></a> module is another new use of
ABCs, defining Python’s “numeric tower”. Also note the new
<a class="reference internal" href="../library/fractions.html#module-fractions" title="fractions: Rational numbers."><code class="xref py py-mod docutils literal notranslate">fractions</code></a> module which implements <a class="reference internal" href="../library/numbers.html#numbers.Rational" title="numbers.Rational"><code class="xref py py-class docutils literal notranslate">numbers.Rational</code></a>.</li>
</ul>
</section>
<section id="library-changes">
<h2>Library Changes<a class="headerlink" href="#library-changes" title="Permalink to this headline">¶</a></h2>
Due to time constraints, this document does not exhaustively cover the
very extensive changes to the standard library. <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3108">PEP 3108</a> is the
reference for the major changes to the library. Here’s a capsule
review:
<ul>
<li>Many old modules were removed. Some, like <code class="xref py py-mod docutils literal notranslate">gopherlib</code> (no
longer used) and <code class="xref py py-mod docutils literal notranslate">md5</code> (replaced by <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>), were
already deprecated by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0004">PEP 4</a>. Others were removed as a result
of the removal of support for various platforms such as Irix, BeOS
and Mac OS 9 (see <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0011">PEP 11</a>). Some modules were also selected for
removal in Python 3.0 due to lack of use or because a better
replacement exists. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3108">PEP 3108</a> for an exhaustive list.</li>
<li>The <code class="xref py py-mod docutils literal notranslate">bsddb3</code> package was removed because its presence in the
core standard library has proved over time to be a particular burden
for the core developers due to testing instability and Berkeley DB’s
release schedule. However, the package is alive and well,
externally maintained at <a class="reference external" href="https://www.jcea.es/programacion/pybsddb.htm">https://www.jcea.es/programacion/pybsddb.htm</a>.</li>
<li>Some modules were renamed because their old name disobeyed
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0008">PEP 8</a>, or for various other reasons. Here’s the list:
<table class="docutils align-default">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Old Name</th>
<th class="head">New Name</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td>_winreg</td>
<td>winreg</td>
</tr>
<tr class="row-odd"><td>ConfigParser</td>
<td>configparser</td>
</tr>
<tr class="row-even"><td>copy_reg</td>
<td>copyreg</td>
</tr>
<tr class="row-odd"><td>Queue</td>
<td>queue</td>
</tr>
<tr class="row-even"><td>SocketServer</td>
<td>socketserver</td>
</tr>
<tr class="row-odd"><td>markupbase</td>
<td>_markupbase</td>
</tr>
<tr class="row-even"><td>repr</td>
<td>reprlib</td>
</tr>
<tr class="row-odd"><td>test.test_support</td>
<td>test.support</td>
</tr>
</tbody>
</table>
</li>
<li>A common pattern in Python 2.x is to have one version of a module
implemented in pure Python, with an optional accelerated version
implemented as a C extension; for example, <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> and
<code class="xref py py-mod docutils literal notranslate">cPickle</code>. This places the burden of importing the accelerated
version and falling back on the pure Python version on each user of
these modules. In Python 3.0, the accelerated versions are
considered implementation details of the pure Python versions.
Users should always import the standard version, which attempts to
import the accelerated version and falls back to the pure Python
version. The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> / <code class="xref py py-mod docutils literal notranslate">cPickle</code> pair received this
treatment. The <a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate">profile</code></a> module is on the list for 3.1. The
<code class="xref py py-mod docutils literal notranslate">StringIO</code> module has been turned into a class in the <a class="reference internal" href="../library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal notranslate">io</code></a>
module.</li>
<li>Some related modules have been grouped into packages, and usually
the submodule names have been simplified. The resulting new
packages are:
<ul class="simple">
<li><a class="reference internal" href="../library/dbm.html#module-dbm" title="dbm: Interfaces to various Unix &quot;database&quot; formats."><code class="xref py py-mod docutils literal notranslate">dbm</code></a> (<code class="xref py py-mod docutils literal notranslate">anydbm</code>, <code class="xref py py-mod docutils literal notranslate">dbhash</code>, <a class="reference internal" href="../library/dbm.html#module-dbm" title="dbm: Interfaces to various Unix &quot;database&quot; formats."><code class="xref py py-mod docutils literal notranslate">dbm</code></a>,
<code class="xref py py-mod docutils literal notranslate">dumbdbm</code>, <code class="xref py py-mod docutils literal notranslate">gdbm</code>, <code class="xref py py-mod docutils literal notranslate">whichdb</code>).</li>
<li><a class="reference internal" href="../library/html.html#module-html" title="html: Helpers for manipulating HTML."><code class="xref py py-mod docutils literal notranslate">html</code></a> (<code class="xref py py-mod docutils literal notranslate">HTMLParser</code>, <code class="xref py py-mod docutils literal notranslate">htmlentitydefs</code>).</li>
<li><a class="reference internal" href="../library/http.html#module-http" title="http: HTTP status codes and messages"><code class="xref py py-mod docutils literal notranslate">http</code></a> (<code class="xref py py-mod docutils literal notranslate">httplib</code>, <code class="xref py py-mod docutils literal notranslate">BaseHTTPServer</code>,
<code class="xref py py-mod docutils literal notranslate">CGIHTTPServer</code>, <code class="xref py py-mod docutils literal notranslate">SimpleHTTPServer</code>, <code class="xref py py-mod docutils literal notranslate">Cookie</code>,
<code class="xref py py-mod docutils literal notranslate">cookielib</code>).</li>
<li><a class="reference internal" href="../library/tkinter.html#module-tkinter" title="tkinter: Interface to Tcl/Tk for graphical user interfaces"><code class="xref py py-mod docutils literal notranslate">tkinter</code></a> (all <code class="xref py py-mod docutils literal notranslate">Tkinter</code>-related modules except
<a class="reference internal" href="../library/turtle.html#module-turtle" title="turtle: An educational framework for simple graphics applications"><code class="xref py py-mod docutils literal notranslate">turtle</code></a>). The target audience of <a class="reference internal" href="../library/turtle.html#module-turtle" title="turtle: An educational framework for simple graphics applications"><code class="xref py py-mod docutils literal notranslate">turtle</code></a> doesn’t
really care about <a class="reference internal" href="../library/tkinter.html#module-tkinter" title="tkinter: Interface to Tcl/Tk for graphical user interfaces"><code class="xref py py-mod docutils literal notranslate">tkinter</code></a>. Also note that as of Python
2.6, the functionality of <a class="reference internal" href="../library/turtle.html#module-turtle" title="turtle: An educational framework for simple graphics applications"><code class="xref py py-mod docutils literal notranslate">turtle</code></a> has been greatly enhanced.</li>
<li><a class="reference internal" href="../library/urllib.html#module-urllib" title="urllib"><code class="xref py py-mod docutils literal notranslate">urllib</code></a> (<a class="reference internal" href="../library/urllib.html#module-urllib" title="urllib"><code class="xref py py-mod docutils literal notranslate">urllib</code></a>, <code class="xref py py-mod docutils literal notranslate">urllib2</code>, <code class="xref py py-mod docutils literal notranslate">urlparse</code>,
<code class="xref py py-mod docutils literal notranslate">robotparse</code>).</li>
<li><code class="xref py py-mod docutils literal notranslate">xmlrpc</code> (<code class="xref py py-mod docutils literal notranslate">xmlrpclib</code>, <code class="xref py py-mod docutils literal notranslate">DocXMLRPCServer</code>,
<code class="xref py py-mod docutils literal notranslate">SimpleXMLRPCServer</code>).</li>
</ul>
</li>
</ul>
Some other changes to standard library modules, not covered by
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-3108">PEP 3108</a>:
<ul class="simple">
<li>Killed <code class="xref py py-mod docutils literal notranslate">sets</code>. Use the built-in <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-func docutils literal notranslate">set()</code></a> class.</li>
<li>Cleanup of the <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate">sys</code></a> module: removed <code class="xref py py-func docutils literal notranslate">sys.exitfunc()</code>,
<code class="xref py py-func docutils literal notranslate">sys.exc_clear()</code>, <code class="xref py py-data docutils literal notranslate">sys.exc_type</code>, <code class="xref py py-data docutils literal notranslate">sys.exc_value</code>,
<code class="xref py py-data docutils literal notranslate">sys.exc_traceback</code>. (Note that <a class="reference internal" href="../library/sys.html#sys.last_type" title="sys.last_type"><code class="xref py py-data docutils literal notranslate">sys.last_type</code></a>
etc. remain.)</li>
<li>Cleanup of the <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> type: the <code class="xref py py-meth docutils literal notranslate">read()</code> and
<code class="xref py py-meth docutils literal notranslate">write()</code> methods are gone; use <code class="xref py py-meth docutils literal notranslate">fromfile()</code> and
<code class="xref py py-meth docutils literal notranslate">tofile()</code> instead. Also, the <code class="docutils literal notranslate">'c'</code> typecode for array is
gone – use either <code class="docutils literal notranslate">'b'</code> for bytes or <code class="docutils literal notranslate">'u'</code> for Unicode
characters.</li>
<li>Cleanup 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: removed
<code class="xref py py-func docutils literal notranslate">sequenceIncludes()</code> and <code class="xref py py-func docutils literal notranslate">isCallable()</code>.</li>
<li>Cleanup of the <code class="xref py py-mod docutils literal notranslate">thread</code> module: <code class="xref py py-func docutils literal notranslate">acquire_lock()</code> and
<code class="xref py py-func docutils literal notranslate">release_lock()</code> are gone; use <code class="xref py py-func docutils literal notranslate">acquire()</code> and
<code class="xref py py-func docutils literal notranslate">release()</code> instead.</li>
<li>Cleanup of 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: removed the <code class="xref py py-func docutils literal notranslate">jumpahead()</code> API.</li>
<li>The <code class="xref py py-mod docutils literal notranslate">new</code> module is gone.</li>
<li>The functions <code class="xref py py-func docutils literal notranslate">os.tmpnam()</code>, <code class="xref py py-func docutils literal notranslate">os.tempnam()</code> and
<code class="xref py py-func docutils literal notranslate">os.tmpfile()</code> have been removed in favor of the <a class="reference internal" href="../library/tempfile.html#module-tempfile" title="tempfile: Generate temporary files and directories."><code class="xref py py-mod docutils literal notranslate">tempfile</code></a>
module.</li>
<li>The <a class="reference internal" href="../library/tokenize.html#module-tokenize" title="tokenize: Lexical scanner for Python source code."><code class="xref py py-mod docutils literal notranslate">tokenize</code></a> module has been changed to work with bytes. The
main entry point is now <a class="reference internal" href="../library/tokenize.html#tokenize.tokenize" title="tokenize.tokenize"><code class="xref py py-func docutils literal notranslate">tokenize.tokenize()</code></a>, instead of
generate_tokens.</li>
<li><code class="xref py py-data docutils literal notranslate">string.letters</code> and its friends (<code class="xref py py-data docutils literal notranslate">string.lowercase</code> and
<code class="xref py py-data docutils literal notranslate">string.uppercase</code>) are gone. Use
<a class="reference internal" href="../library/string.html#string.ascii_letters" title="string.ascii_letters"><code class="xref py py-data docutils literal notranslate">string.ascii_letters</code></a> etc. instead. (The reason for the
removal is that <code class="xref py py-data docutils literal notranslate">string.letters</code> and friends had
locale-specific behavior, which is a bad idea for such
attractively named global “constants”.)</li>
<li>Renamed module <code class="xref py py-mod docutils literal notranslate">__builtin__</code> to <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate">builtins</code></a> (removing the
underscores, adding an ‘s’). The <code class="xref py py-data docutils literal notranslate">__builtins__</code> variable
found in most global namespaces is unchanged. To modify a builtin,
you should use <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate">builtins</code></a>, not <code class="xref py py-data docutils literal notranslate">__builtins__</code>!</li>
</ul>
</section>
<section id="pep-3101-a-new-approach-to-string-formatting">
<h2><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3101">PEP 3101</a>: A New Approach To String Formatting<a class="headerlink" href="#pep-3101-a-new-approach-to-string-formatting" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>A new system for built-in string formatting operations replaces the
<code class="docutils literal notranslate">%</code> string formatting operator. (However, the <code class="docutils literal notranslate">%</code> operator is
still supported; it will be deprecated in Python 3.1 and removed
from the language at some later time.) Read <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3101">PEP 3101</a> for the full
scoop.</li>
</ul>
</section>
<section id="changes-to-exceptions">
<h2>Changes To Exceptions<a class="headerlink" href="#changes-to-exceptions" title="Permalink to this headline">¶</a></h2>
The APIs for raising and catching exception have been cleaned up and
new powerful features added:
<ul>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0352">PEP 352</a>: All exceptions must be derived (directly or indirectly)
from <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate">BaseException</code></a>. This is the root of the exception
hierarchy. This is not new as a recommendation, but the
requirement to inherit from <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate">BaseException</code></a> is new. (Python
2.6 still allowed classic classes to be raised, and placed no
restriction on what you can catch.) As a consequence, string
exceptions are finally truly and utterly dead.</li>
<li>Almost all exceptions should actually derive from <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a>;
<a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate">BaseException</code></a> should only be used as a base class for
exceptions that should only be handled at the top level, such as
<a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate">SystemExit</code></a> or <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a>. The recommended
idiom for handling all exceptions except for this latter category is
to use <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate">Exception</code></a>.</li>
<li><code class="xref py py-exc docutils literal notranslate">StandardError</code> was removed.</li>
<li>Exceptions no longer behave as sequences. Use the <code class="xref py py-attr docutils literal notranslate">args</code>
attribute instead.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3109">PEP 3109</a>: Raising exceptions. You must now use <code class="samp docutils literal notranslate">raise
Exception(args)</code> instead of <code class="samp docutils literal notranslate">raise Exception, args</code>.
Additionally, you can no longer explicitly specify a traceback;
instead, if you have to do this, you can assign directly to the
<code class="xref py py-attr docutils literal notranslate">__traceback__</code> attribute (see below).</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3110">PEP 3110</a>: Catching exceptions. You must now use
<code class="samp docutils literal notranslate">except SomeException as variable</code> instead
of <code class="samp docutils literal notranslate">except SomeException, variable</code>. Moreover, the
variable is explicitly deleted when the <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> block
is left.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3134">PEP 3134</a>: Exception chaining. There are two cases: implicit
chaining and explicit chaining. Implicit chaining happens when an
exception is raised in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> or <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a>
handler block. This usually happens due to a bug in the handler
block; we call this a secondary exception. In this case, the
original exception (that was being handled) is saved as the
<code class="xref py py-attr docutils literal notranslate">__context__</code> attribute of the secondary exception.
Explicit chaining is invoked with this syntax:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>raise SecondaryException() from primary_exception
</pre></div>
</div>
(where primary_exception is any expression that produces an
exception object, probably an exception that was previously caught).
In this case, the primary exception is stored on the
<code class="xref py py-attr docutils literal notranslate">__cause__</code> attribute of the secondary exception. The
traceback printed when an unhandled exception occurs walks the chain
of <code class="xref py py-attr docutils literal notranslate">__cause__</code> and <code class="xref py py-attr docutils literal notranslate">__context__</code> attributes and prints a
separate traceback for each component of the chain, with the primary
exception at the top. (Java users may recognize this behavior.)
</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3134">PEP 3134</a>: Exception objects now store their traceback as the
<code class="xref py py-attr docutils literal notranslate">__traceback__</code> attribute. This means that an exception
object now contains all the information pertaining to an exception,
and there are fewer reasons to use <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate">sys.exc_info()</code></a> (though the
latter is not removed).</li>
<li>A few exception messages are improved when Windows fails to load an
extension module. For example, <code class="docutils literal notranslate">error code 193</code> is now <code class="docutils literal notranslate">%1 is
not a valid Win32 application</code>. Strings now deal with non-English
locales.</li>
</ul>
</section>
<section id="miscellaneous-other-changes">
<h2>Miscellaneous Other Changes<a class="headerlink" href="#miscellaneous-other-changes" title="Permalink to this headline">¶</a></h2>
<section id="operators-and-special-methods">
<h3>Operators And Special Methods<a class="headerlink" href="#operators-and-special-methods" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><code class="docutils literal notranslate">!=</code> now returns the opposite of <code class="docutils literal notranslate">==</code>, unless <code class="docutils literal notranslate">==</code> returns
<a class="reference internal" href="../library/constants.html#NotImplemented" title="NotImplemented"><code class="xref py py-data docutils literal notranslate">NotImplemented</code></a>.</li>
<li>The concept of “unbound methods” has been removed from the language.
When referencing a method as a class attribute, you now get a plain
function object.</li>
<li><code class="xref py py-meth docutils literal notranslate">__getslice__()</code>, <code class="xref py py-meth docutils literal notranslate">__setslice__()</code> and <code class="xref py py-meth docutils literal notranslate">__delslice__()</code>
were killed. The syntax <code class="docutils literal notranslate">a[i:j]</code> now translates to
<code class="docutils literal notranslate">a.__getitem__(slice(i, j))</code> (or <code class="xref py py-meth docutils literal notranslate">__setitem__()</code> or
<code class="xref py py-meth docutils literal notranslate">__delitem__()</code>, when used as an assignment or deletion target,
respectively).</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3114">PEP 3114</a>: the standard <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-meth docutils literal notranslate">next()</code></a> method has been renamed to
<a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a>.</li>
<li>The <code class="xref py py-meth docutils literal notranslate">__oct__()</code> and <code class="xref py py-meth docutils literal notranslate">__hex__()</code> special methods are removed
– <a class="reference internal" href="../library/functions.html#oct" title="oct"><code class="xref py py-func docutils literal notranslate">oct()</code></a> and <a class="reference internal" href="../library/functions.html#hex" title="hex"><code class="xref py py-func docutils literal notranslate">hex()</code></a> use <code class="xref py py-meth docutils literal notranslate">__index__()</code> now to convert
the argument to an integer.</li>
<li>Removed support for <code class="xref py py-attr docutils literal notranslate">__members__</code> and <code class="xref py py-attr docutils literal notranslate">__methods__</code>.</li>
<li>The function attributes named <code class="xref py py-attr docutils literal notranslate">func_X</code> have been renamed to
use the <code class="xref py py-data docutils literal notranslate">__X__</code> form, freeing up these names in the function
attribute namespace for user-defined attributes. To wit,
<code class="xref py py-attr docutils literal notranslate">func_closure</code>, <code class="xref py py-attr docutils literal notranslate">func_code</code>, <code class="xref py py-attr docutils literal notranslate">func_defaults</code>,
<code class="xref py py-attr docutils literal notranslate">func_dict</code>, <code class="xref py py-attr docutils literal notranslate">func_doc</code>, <code class="xref py py-attr docutils literal notranslate">func_globals</code>,
<code class="xref py py-attr docutils literal notranslate">func_name</code> were renamed to <code class="xref py py-attr docutils literal notranslate">__closure__</code>,
<code class="xref py py-attr docutils literal notranslate">__code__</code>, <code class="xref py py-attr docutils literal notranslate">__defaults__</code>, <a class="reference internal" href="../library/stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal notranslate">__dict__</code></a>,
<code class="xref py py-attr docutils literal notranslate">__doc__</code>, <code class="xref py py-attr docutils literal notranslate">__globals__</code>, <a class="reference internal" href="../library/stdtypes.html#definition.__name__" title="definition.__name__"><code class="xref py py-attr docutils literal notranslate">__name__</code></a>,
respectively.</li>
<li><code class="xref py py-meth docutils literal notranslate">__nonzero__()</code> is now <code class="xref py py-meth docutils literal notranslate">__bool__()</code>.</li>
</ul>
</section>
<section id="builtins">
<h3>Builtins<a class="headerlink" href="#builtins" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3135">PEP 3135</a>: New <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate">super()</code></a>. You can now invoke <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate">super()</code></a>
without arguments and (assuming this is in a regular instance method
defined inside a <a class="reference internal" href="../reference/compound_stmts.html#class"><code class="xref std std-keyword docutils literal notranslate">class</code></a> statement) the right class and
instance will automatically be chosen. With arguments, the behavior
of <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate">super()</code></a> is unchanged.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3111">PEP 3111</a>: <code class="xref py py-func docutils literal notranslate">raw_input()</code> was renamed to <a class="reference internal" href="../library/functions.html#input" title="input"><code class="xref py py-func docutils literal notranslate">input()</code></a>. That
is, the new <a class="reference internal" href="../library/functions.html#input" title="input"><code class="xref py py-func docutils literal notranslate">input()</code></a> function reads a line from
<a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal notranslate">sys.stdin</code></a> and returns it with the trailing newline stripped.
It raises <a class="reference internal" href="../library/exceptions.html#EOFError" title="EOFError"><code class="xref py py-exc docutils literal notranslate">EOFError</code></a> if the input is terminated prematurely.
To get the old behavior of <a class="reference internal" href="../library/functions.html#input" title="input"><code class="xref py py-func docutils literal notranslate">input()</code></a>, use <code class="docutils literal notranslate">eval(input())</code>.</li>
<li>A new built-in function <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-func docutils literal notranslate">next()</code></a> was added to call the
<a class="reference internal" href="../library/stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a> method on an object.</li>
<li>The <a class="reference internal" href="../library/functions.html#round" title="round"><code class="xref py py-func docutils literal notranslate">round()</code></a> function rounding strategy and return type have
changed. Exact halfway cases are now rounded to the nearest even
result instead of away from zero. (For example, <code class="docutils literal notranslate">round(2.5)</code> now
returns <code class="docutils literal notranslate">2</code> rather than <code class="docutils literal notranslate">3</code>.) <code class="docutils literal notranslate">round(x[, n])</code> now
delegates to <code class="docutils literal notranslate">x.__round__([n])</code> instead of always returning a
float. It generally returns an integer when called with a single
argument and a value of the same type as <code class="docutils literal notranslate">x</code> when called with two
arguments.</li>
<li>Moved <code class="xref py py-func docutils literal notranslate">intern()</code> to <a class="reference internal" href="../library/sys.html#sys.intern" title="sys.intern"><code class="xref py py-func docutils literal notranslate">sys.intern()</code></a>.</li>
<li>Removed: <code class="xref py py-func docutils literal notranslate">apply()</code>. Instead of <code class="docutils literal notranslate">apply(f, args)</code> use
<code class="docutils literal notranslate">f(*args)</code>.</li>
<li>Removed <a class="reference internal" href="../library/functions.html#callable" title="callable"><code class="xref py py-func docutils literal notranslate">callable()</code></a>. Instead of <code class="docutils literal notranslate">callable(f)</code> you can use
<code class="docutils literal notranslate">isinstance(f, collections.Callable)</code>. The <code class="xref py py-func docutils literal notranslate">operator.isCallable()</code>
function is also gone.</li>
<li>Removed <code class="xref py py-func docutils literal notranslate">coerce()</code>. This function no longer serves a purpose
now that classic classes are gone.</li>
<li>Removed <code class="xref py py-func docutils literal notranslate">execfile()</code>. Instead of <code class="docutils literal notranslate">execfile(fn)</code> use
<code class="docutils literal notranslate">exec(open(fn).read())</code>.</li>
<li>Removed the <code class="xref py py-class docutils literal notranslate">file</code> type. Use <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a>. There are now several
different kinds of streams that open can return in the <a class="reference internal" href="../library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal notranslate">io</code></a> module.</li>
<li>Removed <code class="xref py py-func docutils literal notranslate">reduce()</code>. Use <a class="reference internal" href="../library/functools.html#functools.reduce" title="functools.reduce"><code class="xref py py-func docutils literal notranslate">functools.reduce()</code></a> if you really
need it; however, 99 percent of the time an explicit <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a>
loop is more readable.</li>
<li>Removed <code class="xref py py-func docutils literal notranslate">reload()</code>. Use <a class="reference internal" href="../library/imp.html#imp.reload" title="imp.reload"><code class="xref py py-func docutils literal notranslate">imp.reload()</code></a>.</li>
<li>Removed. <code class="xref py py-meth docutils literal notranslate">dict.has_key()</code> – use the <a class="reference internal" href="../reference/expressions.html#in"><code class="xref std std-keyword docutils literal notranslate">in</code></a> operator
instead.</li>
</ul>
</section>
</section>
<section id="build-and-c-api-changes">
<h2>Build and C API Changes<a class="headerlink" href="#build-and-c-api-changes" title="Permalink to this headline">¶</a></h2>
Due to time constraints, here is a very incomplete list of changes
to the C API.
<ul class="simple">
<li>Support for several platforms was dropped, including but not limited
to Mac OS 9, BeOS, RISCOS, Irix, and Tru64.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3118">PEP 3118</a>: New Buffer API.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3121">PEP 3121</a>: Extension Module Initialization &amp; Finalization.</li>
<li><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3123">PEP 3123</a>: Making <a class="reference internal" href="../c-api/structures.html#c.PyObject_HEAD" title="PyObject_HEAD"><code class="xref c c-macro docutils literal notranslate">PyObject_HEAD</code></a> conform to standard C.</li>
<li>No more C API support for restricted execution.</li>
<li><code class="xref c c-func docutils literal notranslate">PyNumber_Coerce()</code>, <code class="xref c c-func docutils literal notranslate">PyNumber_CoerceEx()</code>,
<code class="xref c c-func docutils literal notranslate">PyMember_Get()</code>, and <code class="xref c c-func docutils literal notranslate">PyMember_Set()</code> C APIs are removed.</li>
<li>New C API <a class="reference internal" href="../c-api/import.html#c.PyImport_ImportModuleNoBlock" title="PyImport_ImportModuleNoBlock"><code class="xref c c-func docutils literal notranslate">PyImport_ImportModuleNoBlock()</code></a>, works like
<a class="reference internal" href="../c-api/import.html#c.PyImport_ImportModule" title="PyImport_ImportModule"><code class="xref c c-func docutils literal notranslate">PyImport_ImportModule()</code></a> but won’t block on the import lock
(returning an error instead).</li>
<li>Renamed the boolean conversion C-level slot and method:
<code class="docutils literal notranslate">nb_nonzero</code> is now <code class="docutils literal notranslate">nb_bool</code>.</li>
<li>Removed <code class="xref c c-macro docutils literal notranslate">METH_OLDARGS</code> and <code class="xref c c-macro docutils literal notranslate">WITH_CYCLE_GC</code> from the C API.</li>
</ul>
</section>
<section id="performance">
<h2>Performance<a class="headerlink" href="#performance" title="Permalink to this headline">¶</a></h2>
The net result of the 3.0 generalizations is that Python 3.0 runs the
pystone benchmark around 10% slower than Python 2.5. Most likely the
biggest cause is the removal of special-casing for small integers.
There’s room for improvement, but it will happen after 3.0 is
released!
</section>
<section id="porting-to-python-3-0">
<h2>Porting To Python 3.0<a class="headerlink" href="#porting-to-python-3-0" title="Permalink to this headline">¶</a></h2>
For porting existing Python 2.5 or 2.6 source code to Python 3.0, the
best strategy is the following:
<ol class="arabic simple" start="0">
<li>(Prerequisite:) Start with excellent test coverage.</li>
<li>Port to Python 2.6. This should be no more work than the average
port from Python 2.x to Python 2.(x+1). Make sure all your tests
pass.</li>
<li>(Still using 2.6:) Turn on the <code class="xref std std-option docutils literal notranslate">-3</code> command line switch.
This enables warnings about features that will be removed (or
change) in 3.0. Run your test suite again, and fix code that you
get warnings about until there are no warnings left, and all your
tests still pass.</li>
<li>Run the <code class="docutils literal notranslate">2to3</code> source-to-source translator over your source code
tree. (See <a class="reference internal" href="../library/2to3.html#to3-reference">2to3 — Automated Python 2 to 3 code translation</a> for more on this tool.) Run the
result of the translation under Python 3.0. Manually fix up any
remaining issues, fixing problems until all tests pass again.</li>
</ol>
It is not recommended to try to write source code that runs unchanged
under both Python 2.6 and 3.0; you’d have to use a very contorted
coding style, e.g. avoiding <code class="docutils literal notranslate">print</code> statements, metaclasses,
and much more. If you are maintaining a library that needs to support
both Python 2.6 and Python 3.0, the best approach is to modify step 3
above by editing the 2.6 version of the source code and running the
<code class="docutils literal notranslate">2to3</code> translator again, rather than editing the 3.0 version of the
source code.
For porting C extensions to Python 3.0, please see <a class="reference internal" href="../howto/cporting.html#cporting-howto">Porting Extension Modules to Python 3</a>.
</section>
</section>

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 <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 3.0</a><ul>
<li><a class="reference internal" href="#common-stumbling-blocks">Common Stumbling Blocks</a><ul>
<li><a class="reference internal" href="#print-is-a-function">Print Is A Function</a></li>
<li><a class="reference internal" href="#views-and-iterators-instead-of-lists">Views And Iterators Instead Of Lists</a></li>
<li><a class="reference internal" href="#ordering-comparisons">Ordering Comparisons</a></li>
<li><a class="reference internal" href="#integers">Integers</a></li>
<li><a class="reference internal" href="#text-vs-data-instead-of-unicode-vs-8-bit">Text Vs. Data Instead Of Unicode Vs. 8-bit</a></li>
</ul>
</li>
<li><a class="reference internal" href="#overview-of-syntax-changes">Overview Of Syntax Changes</a><ul>
<li><a class="reference internal" href="#new-syntax">New Syntax</a></li>
<li><a class="reference internal" href="#changed-syntax">Changed Syntax</a></li>
<li><a class="reference internal" href="#removed-syntax">Removed Syntax</a></li>
</ul>
</li>
<li><a class="reference internal" href="#changes-already-present-in-python-2-6">Changes Already Present In Python 2.6</a></li>
<li><a class="reference internal" href="#library-changes">Library Changes</a></li>
<li><a class="reference internal" href="#pep-3101-a-new-approach-to-string-formatting">PEP 3101: A New Approach To String Formatting</a></li>
<li><a class="reference internal" href="#changes-to-exceptions">Changes To Exceptions</a></li>
<li><a class="reference internal" href="#miscellaneous-other-changes">Miscellaneous Other Changes</a><ul>
<li><a class="reference internal" href="#operators-and-special-methods">Operators And Special Methods</a></li>
<li><a class="reference internal" href="#builtins">Builtins</a></li>
</ul>
</li>
<li><a class="reference internal" href="#build-and-c-api-changes">Build and C API Changes</a></li>
<li><a class="reference internal" href="#performance">Performance</a></li>
<li><a class="reference internal" href="#porting-to-python-3-0">Porting To Python 3.0</a></li>
</ul>
</li>
</ul>

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