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 <ul>
<li><a class="reference internal" href="#">What’s New In Python 3.1</a><ul>
<li><a class="reference internal" href="#pep-372-ordered-dictionaries">PEP 372: Ordered Dictionaries</a></li>
<li><a class="reference internal" href="#pep-378-format-specifier-for-thousands-separator">PEP 378: Format Specifier for Thousands Separator</a></li>
<li><a class="reference internal" href="#other-language-changes">Other Language Changes</a></li>
<li><a class="reference internal" href="#new-improved-and-deprecated-modules">New, Improved, and Deprecated Modules</a></li>
<li><a class="reference internal" href="#optimizations">Optimizations</a></li>
<li><a class="reference internal" href="#idle">IDLE</a></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="#porting-to-python-3-1">Porting to Python 3.1</a></li>
</ul>
</li>
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 <section id="what-s-new-in-python-3-1">
<h1>What’s New In Python 3.1<a class="headerlink" href="#what-s-new-in-python-3-1" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd">Raymond Hettinger
</dd>
</dl>
This article explains the new features in Python 3.1, compared to 3.0.
Python 3.1 was released on June 27, 2009.
<section id="pep-372-ordered-dictionaries">
<h2>PEP 372: Ordered Dictionaries<a class="headerlink" href="#pep-372-ordered-dictionaries" title="Permalink to this headline">¶</a></h2>
Regular Python dictionaries iterate over key/value pairs in arbitrary order.
Over the years, a number of authors have written alternative implementations
that remember the order that the keys were originally inserted. Based on
the experiences from those implementations, a new
<a class="reference internal" href="../library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">collections.OrderedDict</code></a> class has been introduced.
The OrderedDict API is substantially the same as regular dictionaries
but will iterate over keys and values in a guaranteed order depending on
when a key was first inserted. If a new entry overwrites an existing entry,
the original insertion position is left unchanged. Deleting an entry and
reinserting it will move it to the end.
The standard library now supports use of ordered dictionaries in several
modules. The <a class="reference internal" href="../library/configparser.html#module-configparser" title="configparser: Configuration file parser."><code class="xref py py-mod docutils literal notranslate">configparser</code></a> module uses them by default. This lets
configuration files be read, modified, and then written back in their original
order. The _asdict() method for <a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">collections.namedtuple()</code></a> now
returns an ordered dictionary with the values appearing in the same order as
the underlying tuple indices. The <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate">json</code></a> module is being built-out with
an object_pairs_hook to allow OrderedDicts to be built by the decoder.
Support was also added for third-party tools like <a class="reference external" href="https://pyyaml.org/">PyYAML</a>.
<div class="admonition seealso">
See also
<dl class="simple">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0372">PEP 372</a> - Ordered Dictionaries</dt><dd>PEP written by Armin Ronacher and Raymond Hettinger. Implementation
written by Raymond Hettinger.
</dd>
</dl>
</div>
</section>
<section id="pep-378-format-specifier-for-thousands-separator">
<h2>PEP 378: Format Specifier for Thousands Separator<a class="headerlink" href="#pep-378-format-specifier-for-thousands-separator" title="Permalink to this headline">¶</a></h2>
The built-in <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate">format()</code></a> function and the <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate">str.format()</code></a> method use
a mini-language that now includes a simple, non-locale aware way to format
a number with a thousands separator. That provides a way to humanize a
program’s output, improving its professional appearance and readability:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; format(1234567, &#39;,d&#39;)
&#39;1,234,567&#39;
&gt;&gt;&gt; format(1234567.89, &#39;,.2f&#39;)
&#39;1,234,567.89&#39;
&gt;&gt;&gt; format(12345.6 + 8901234.12j, &#39;,f&#39;)
&#39;12,345.600000+8,901,234.120000j&#39;
&gt;&gt;&gt; format(Decimal(&#39;1234567.89&#39;), &#39;,f&#39;)
&#39;1,234,567.89&#39;
</pre></div>
</div>
The supported types are <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>, <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate">complex</code></a>
and <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">decimal.Decimal</code></a>.
Discussions are underway about how to specify alternative separators
like dots, spaces, apostrophes, or underscores. Locale-aware applications
should use the existing n format specifier which already has some support
for thousands separators.
<div class="admonition seealso">
See also
<dl class="simple">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0378">PEP 378</a> - Format Specifier for Thousands Separator</dt><dd>PEP written by Raymond Hettinger and implemented by Eric Smith and
Mark Dickinson.
</dd>
</dl>
</div>
</section>
<section id="other-language-changes">
<h2>Other Language Changes<a class="headerlink" href="#other-language-changes" title="Permalink to this headline">¶</a></h2>
Some smaller changes made to the core Python language are:
<ul>
<li>Directories and zip archives containing a <code class="file docutils literal notranslate">__main__.py</code>
file can now be executed directly by passing their name to the
interpreter. The directory/zipfile is automatically inserted as the
first entry in sys.path. (Suggestion and initial patch by Andy Chu;
revised patch by Phillip J. Eby and Nick Coghlan; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1739468">bpo-1739468</a>.)</li>
<li>The <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> type gained a <code class="docutils literal notranslate">bit_length</code> method that returns the
number of bits necessary to represent its argument in binary:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; n = 37
&gt;&gt;&gt; bin(37)
&#39;0b100101&#39;
&gt;&gt;&gt; n.bit_length()
6
&gt;&gt;&gt; n = 2**123-1
&gt;&gt;&gt; n.bit_length()
123
&gt;&gt;&gt; (n+1).bit_length()
124
</pre></div>
</div>
(Contributed by Fredrik Johansson, Victor Stinner, Raymond Hettinger,
and Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=3439">bpo-3439</a>.)
</li>
<li>The fields in <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate">format()</code></a> strings can now be automatically
numbered:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; &#39;Sir {} of {}&#39;.format(&#39;Gallahad&#39;, &#39;Camelot&#39;)
&#39;Sir Gallahad of Camelot&#39;
</pre></div>
</div>
Formerly, the string would have required numbered fields such as:
<code class="docutils literal notranslate">'Sir {0} of {1}'</code>.
(Contributed by Eric Smith; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5237">bpo-5237</a>.)
</li>
<li>The <code class="xref py py-func docutils literal notranslate">string.maketrans()</code> function is deprecated and is replaced by new
static methods, <a class="reference internal" href="../library/stdtypes.html#bytes.maketrans" title="bytes.maketrans"><code class="xref py py-meth docutils literal notranslate">bytes.maketrans()</code></a> and <a class="reference internal" href="../library/stdtypes.html#bytearray.maketrans" title="bytearray.maketrans"><code class="xref py py-meth docutils literal notranslate">bytearray.maketrans()</code></a>.
This change solves the confusion around which types were supported by the
<a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal notranslate">string</code></a> module. Now, <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>, and
<a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a> each have their own maketrans and translate
methods with intermediate translation tables of the appropriate type.
(Contributed by Georg Brandl; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5675">bpo-5675</a>.)
</li>
<li>The syntax of the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement now allows multiple context
managers in a single statement:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; with open(&#39;mylog.txt&#39;) as infile, open(&#39;a.out&#39;, &#39;w&#39;) as outfile:
... for line in infile:
... if &#39;&lt;critical&gt;&#39; in line:
... outfile.write(line)
</pre></div>
</div>
With the new syntax, the <code class="xref py py-func docutils literal notranslate">contextlib.nested()</code> function is no longer
needed and is now deprecated.
(Contributed by Georg Brandl and Mattias Brändström;
<a class="reference external" href="https://codereview.appspot.com/53094">appspot issue 53094</a>.)
</li>
<li><code class="docutils literal notranslate">round(x, n)</code> now returns an integer if x is an integer.
Previously it returned a float:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; round(1123, -2)
1100
</pre></div>
</div>
(Contributed by Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4707">bpo-4707</a>.)
</li>
<li>Python now uses David Gay’s algorithm for finding the shortest floating
point representation that doesn’t change its value. This should help
mitigate some of the confusion surrounding binary floating point
numbers.
The significance is easily seen with a number like <code class="docutils literal notranslate">1.1</code> which does not
have an exact equivalent in binary floating point. Since there is no exact
equivalent, an expression like <code class="docutils literal notranslate">float('1.1')</code> evaluates to the nearest
representable value which is <code class="docutils literal notranslate">0x1.199999999999ap+0</code> in hex or
<code class="docutils literal notranslate">1.100000000000000088817841970012523233890533447265625</code> in decimal. That
nearest value was and still is used in subsequent floating point
calculations.
What is new is how the number gets displayed. Formerly, Python used a
simple approach. The value of <code class="docutils literal notranslate">repr(1.1)</code> was computed as <code class="docutils literal notranslate">format(1.1,
'.17g')</code> which evaluated to <code class="docutils literal notranslate">'1.1000000000000001'</code>. The advantage of
using 17 digits was that it relied on IEEE-754 guarantees to assure that
<code class="docutils literal notranslate">eval(repr(1.1))</code> would round-trip exactly to its original value. The
disadvantage is that many people found the output to be confusing (mistaking
intrinsic limitations of binary floating point representation as being a
problem with Python itself).
The new algorithm for <code class="docutils literal notranslate">repr(1.1)</code> is smarter and returns <code class="docutils literal notranslate">'1.1'</code>.
Effectively, it searches all equivalent string representations (ones that
get stored with the same underlying float value) and returns the shortest
representation.
The new algorithm tends to emit cleaner representations when possible, but
it does not change the underlying values. So, it is still the case that
<code class="docutils literal notranslate">1.1 + 2.2 != 3.3</code> even though the representations may suggest otherwise.
The new algorithm depends on certain features in the underlying floating
point implementation. If the required features are not found, the old
algorithm will continue to be used. Also, the text pickle protocols
assure cross-platform portability by using the old algorithm.
(Contributed by Eric Smith and Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1580">bpo-1580</a>)
</li>
</ul>
</section>
<section id="new-improved-and-deprecated-modules">
<h2>New, Improved, and Deprecated Modules<a class="headerlink" href="#new-improved-and-deprecated-modules" title="Permalink to this headline">¶</a></h2>
<ul>
<li>Added a <a class="reference internal" href="../library/collections.html#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal notranslate">collections.Counter</code></a> class to support convenient
counting of unique items in a sequence or iterable:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Counter([&#39;red&#39;, &#39;blue&#39;, &#39;red&#39;, &#39;green&#39;, &#39;blue&#39;, &#39;blue&#39;])
Counter({&#39;blue&#39;: 3, &#39;red&#39;: 2, &#39;green&#39;: 1})
</pre></div>
</div>
(Contributed by Raymond Hettinger; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1696199">bpo-1696199</a>.)
</li>
<li>Added a new module, <a class="reference internal" href="../library/tkinter.ttk.html#module-tkinter.ttk" title="tkinter.ttk: Tk themed widget set"><code class="xref py py-mod docutils literal notranslate">tkinter.ttk</code></a> for access to the Tk themed widget set.
The basic idea of ttk is to separate, to the extent possible, the code
implementing a widget’s behavior from the code implementing its appearance.
(Contributed by Guilherme Polo; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=2983">bpo-2983</a>.)
</li>
<li>The <a class="reference internal" href="../library/gzip.html#gzip.GzipFile" title="gzip.GzipFile"><code class="xref py py-class docutils literal notranslate">gzip.GzipFile</code></a> and <a class="reference internal" href="../library/bz2.html#bz2.BZ2File" title="bz2.BZ2File"><code class="xref py py-class docutils literal notranslate">bz2.BZ2File</code></a> classes now support
the context management protocol:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # Automatically close file after writing
&gt;&gt;&gt; with gzip.GzipFile(filename, &quot;wb&quot;) as f:
... f.write(b&quot;xxx&quot;)
</pre></div>
</div>
(Contributed by Antoine Pitrou.)
</li>
<li>The <a class="reference internal" href="../library/decimal.html#module-decimal" title="decimal: Implementation of the General Decimal Arithmetic Specification."><code class="xref py py-mod docutils literal notranslate">decimal</code></a> module now supports methods for creating a
decimal object from a binary <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>. The conversion is
exact but can sometimes be surprising:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; Decimal.from_float(1.1)
Decimal(&#39;1.100000000000000088817841970012523233890533447265625&#39;)
</pre></div>
</div>
The long decimal result shows the actual binary fraction being
stored for 1.1. The fraction has many digits because 1.1 cannot
be exactly represented in binary.
(Contributed by Raymond Hettinger and Mark Dickinson.)
</li>
<li>The <a class="reference internal" href="../library/itertools.html#module-itertools" title="itertools: Functions creating iterators for efficient looping."><code class="xref py py-mod docutils literal notranslate">itertools</code></a> module grew two new functions. The
<a class="reference internal" href="../library/itertools.html#itertools.combinations_with_replacement" title="itertools.combinations_with_replacement"><code class="xref py py-func docutils literal notranslate">itertools.combinations_with_replacement()</code></a> function is one of
four for generating combinatorics including permutations and Cartesian
products. The <a class="reference internal" href="../library/itertools.html#itertools.compress" title="itertools.compress"><code class="xref py py-func docutils literal notranslate">itertools.compress()</code></a> function mimics its namesake
from APL. Also, the existing <a class="reference internal" href="../library/itertools.html#itertools.count" title="itertools.count"><code class="xref py py-func docutils literal notranslate">itertools.count()</code></a> function now has
an optional step argument and can accept any type of counting
sequence including <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">fractions.Fraction</code></a> and
<a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">decimal.Decimal</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; [p+q for p,q in combinations_with_replacement(&#39;LOVE&#39;, 2)]
[&#39;LL&#39;, &#39;LO&#39;, &#39;LV&#39;, &#39;LE&#39;, &#39;OO&#39;, &#39;OV&#39;, &#39;OE&#39;, &#39;VV&#39;, &#39;VE&#39;, &#39;EE&#39;]

&gt;&gt;&gt; list(compress(data=range(10), selectors=[0,0,1,1,0,1,0,1,0,0]))
[2, 3, 5, 7]

&gt;&gt;&gt; c = count(start=Fraction(1,2), step=Fraction(1,6))
&gt;&gt;&gt; [next(c), next(c), next(c), next(c)]
[Fraction(1, 2), Fraction(2, 3), Fraction(5, 6), Fraction(1, 1)]
</pre></div>
</div>
(Contributed by Raymond Hettinger.)
</li>
<li><a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">collections.namedtuple()</code></a> now supports a keyword argument
rename which lets invalid fieldnames be automatically converted to
positional names in the form _0, _1, etc. This is useful when
the field names are being created by an external source such as a
CSV header, SQL field list, or user input:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; query = input()
SELECT region, dept, count(*) FROM main GROUPBY region, dept

&gt;&gt;&gt; cursor.execute(query)
&gt;&gt;&gt; query_fields = [desc[0] for desc in cursor.description]
&gt;&gt;&gt; UserQuery = namedtuple(&#39;UserQuery&#39;, query_fields, rename=True)
&gt;&gt;&gt; pprint.pprint([UserQuery(*row) for row in cursor])
[UserQuery(region=&#39;South&#39;, dept=&#39;Shipping&#39;, _2=185),
 UserQuery(region=&#39;North&#39;, dept=&#39;Accounting&#39;, _2=37),
 UserQuery(region=&#39;West&#39;, dept=&#39;Sales&#39;, _2=419)]
</pre></div>
</div>
(Contributed by Raymond Hettinger; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1818">bpo-1818</a>.)
</li>
<li>The <a class="reference internal" href="../library/re.html#re.sub" title="re.sub"><code class="xref py py-func docutils literal notranslate">re.sub()</code></a>, <a class="reference internal" href="../library/re.html#re.subn" title="re.subn"><code class="xref py py-func docutils literal notranslate">re.subn()</code></a> and <a class="reference internal" href="../library/re.html#re.split" title="re.split"><code class="xref py py-func docutils literal notranslate">re.split()</code></a> functions now
accept a flags parameter.
(Contributed by Gregory Smith.)
</li>
<li>The <a class="reference internal" href="../library/logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate">logging</code></a> module now implements a simple <a class="reference internal" href="../library/logging.handlers.html#logging.NullHandler" title="logging.NullHandler"><code class="xref py py-class docutils literal notranslate">logging.NullHandler</code></a>
class for applications that are not using logging but are calling
library code that does. Setting-up a null handler will suppress
spurious warnings such as “No handlers could be found for logger foo”:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; h = logging.NullHandler()
&gt;&gt;&gt; logging.getLogger(&quot;foo&quot;).addHandler(h)
</pre></div>
</div>
(Contributed by Vinay Sajip; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4384">bpo-4384</a>).
</li>
<li>The <a class="reference internal" href="../library/runpy.html#module-runpy" title="runpy: Locate and run Python modules without importing them first."><code class="xref py py-mod docutils literal notranslate">runpy</code></a> module which supports the <code class="docutils literal notranslate">-m</code> command line switch
now supports the execution of packages by looking for and executing
a <code class="docutils literal notranslate">__main__</code> submodule when a package name is supplied.
(Contributed by Andi Vajda; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4195">bpo-4195</a>.)
</li>
<li>The <a class="reference internal" href="../library/pdb.html#module-pdb" title="pdb: The Python debugger for interactive interpreters."><code class="xref py py-mod docutils literal notranslate">pdb</code></a> module can now access and display source code loaded via
<a class="reference internal" href="../library/zipimport.html#module-zipimport" title="zipimport: Support for importing Python modules from ZIP archives."><code class="xref py py-mod docutils literal notranslate">zipimport</code></a> (or any other conformant <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0302">PEP 302</a> loader).
(Contributed by Alexander Belopolsky; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4201">bpo-4201</a>.)
</li>
<li><a class="reference internal" href="../library/functools.html#functools.partial" title="functools.partial"><code class="xref py py-class docutils literal notranslate">functools.partial</code></a> objects can now be pickled.</li>
</ul>
<blockquote>
<div>(Suggested by Antoine Pitrou and Jesse Noller. Implemented by
Jack Diederich; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5228">bpo-5228</a>.)
</div></blockquote>
<ul>
<li>Add <a class="reference internal" href="../library/pydoc.html#module-pydoc" title="pydoc: Documentation generator and online help system."><code class="xref py py-mod docutils literal notranslate">pydoc</code></a> help topics for symbols so that <code class="docutils literal notranslate">help('&#64;')</code>
works as expected in the interactive environment.
(Contributed by David Laban; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4739">bpo-4739</a>.)
</li>
<li>The <a class="reference internal" href="../library/unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><code class="xref py py-mod docutils literal notranslate">unittest</code></a> module now supports skipping individual tests or classes
of tests. And it supports marking a test as an expected failure, a test that
is known to be broken, but shouldn’t be counted as a failure on a
TestResult:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class TestGizmo(unittest.TestCase):

@unittest.skipUnless(sys.platform.startswith(&quot;win&quot;), &quot;requires Windows&quot;)
 def test_gizmo_on_windows(self):
 ...

@unittest.expectedFailure
 def test_gimzo_without_required_library(self):
 ...
</pre></div>
</div>
Also, tests for exceptions have been builtout to work with context managers
using the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def test_division_by_zero(self):
 with self.assertRaises(ZeroDivisionError):
 x / 0
</pre></div>
</div>
In addition, several new assertion methods were added including
<code class="xref py py-func docutils literal notranslate">assertSetEqual()</code>, <code class="xref py py-func docutils literal notranslate">assertDictEqual()</code>,
<code class="xref py py-func docutils literal notranslate">assertDictContainsSubset()</code>, <code class="xref py py-func docutils literal notranslate">assertListEqual()</code>,
<code class="xref py py-func docutils literal notranslate">assertTupleEqual()</code>, <code class="xref py py-func docutils literal notranslate">assertSequenceEqual()</code>,
<code class="xref py py-func docutils literal notranslate">assertRaisesRegexp()</code>, <code class="xref py py-func docutils literal notranslate">assertIsNone()</code>,
and <code class="xref py py-func docutils literal notranslate">assertIsNotNone()</code>.
(Contributed by Benjamin Peterson and Antoine Pitrou.)
</li>
<li>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 has three new constants for the <code class="xref py py-meth docutils literal notranslate">seek()</code>
method <code class="xref py py-data docutils literal notranslate">SEEK_SET</code>, <code class="xref py py-data docutils literal notranslate">SEEK_CUR</code>, and <code class="xref py py-data docutils literal notranslate">SEEK_END</code>.</li>
<li>The <a class="reference internal" href="../library/sys.html#sys.version_info" title="sys.version_info"><code class="xref py py-attr docutils literal notranslate">sys.version_info</code></a> tuple is now a named tuple:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; sys.version_info
sys.version_info(major=3, minor=1, micro=0, releaselevel=&#39;alpha&#39;, serial=2)
</pre></div>
</div>
(Contributed by Ross Light; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4285">bpo-4285</a>.)
</li>
<li>The <a class="reference internal" href="../library/nntplib.html#module-nntplib" title="nntplib: NNTP protocol client (requires sockets). (deprecated)"><code class="xref py py-mod docutils literal notranslate">nntplib</code></a> and <a class="reference internal" href="../library/imaplib.html#module-imaplib" title="imaplib: IMAP4 protocol client (requires sockets)."><code class="xref py py-mod docutils literal notranslate">imaplib</code></a> modules now support IPv6.
(Contributed by Derek Morr; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1655">bpo-1655</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1664">bpo-1664</a>.)
</li>
<li>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> module has been adapted for better interoperability with
Python 2.x when used with protocol 2 or lower. The reorganization of the
standard library changed the formal reference for many objects. For
example, <code class="docutils literal notranslate">__builtin__.set</code> in Python 2 is called <code class="docutils literal notranslate">builtins.set</code> in Python
3. This change confounded efforts to share data between different versions of
Python. But now when protocol 2 or lower is selected, the pickler will
automatically use the old Python 2 names for both loading and dumping. This
remapping is turned-on by default but can be disabled with the fix_imports
option:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = {1, 2, 3}
&gt;&gt;&gt; pickle.dumps(s, protocol=0)
b&#39;c__builtin__\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.&#39;
&gt;&gt;&gt; pickle.dumps(s, protocol=0, fix_imports=False)
b&#39;cbuiltins\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.&#39;
</pre></div>
</div>
An unfortunate but unavoidable side-effect of this change is that protocol 2
pickles produced by Python 3.1 won’t be readable with Python 3.0. The latest
pickle protocol, protocol 3, should be used when migrating data between
Python 3.x implementations, as it doesn’t attempt to remain compatible with
Python 2.x.
(Contributed by Alexandre Vassalotti and Antoine Pitrou, <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=6137">bpo-6137</a>.)
</li>
<li>A new module, <a class="reference internal" href="../library/importlib.html#module-importlib" title="importlib: The implementation of the import machinery."><code class="xref py py-mod docutils literal notranslate">importlib</code></a> was added. It provides a complete, portable,
pure Python reference implementation of the <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</code></a> statement and its
counterpart, the <a class="reference internal" href="../library/functions.html#import__" title="__import__"><code class="xref py py-func docutils literal notranslate">__import__()</code></a> function. It represents a substantial
step forward in documenting and defining the actions that take place during
imports.
(Contributed by Brett Cannon.)
</li>
</ul>
</section>
<section id="optimizations">
<h2>Optimizations<a class="headerlink" href="#optimizations" title="Permalink to this headline">¶</a></h2>
Major performance enhancements have been added:
<ul>
<li>The new I/O library (as defined in <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3116">PEP 3116</a>) was mostly written in
Python and quickly proved to be a problematic bottleneck in Python 3.0.
In Python 3.1, the I/O library has been entirely rewritten in C and is
2 to 20 times faster depending on the task at hand. The pure Python
version is still available for experimentation purposes through
the <code class="docutils literal notranslate">_pyio</code> module.
(Contributed by Amaury Forgeot d’Arc and Antoine Pitrou.)
</li>
<li>Added a heuristic so that tuples and dicts containing only untrackable objects
are not tracked by the garbage collector. This can reduce the size of
collections and therefore the garbage collection overhead on long-running
programs, depending on their particular use of datatypes.
(Contributed by Antoine Pitrou, <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4688">bpo-4688</a>.)
</li>
<li>Enabling a configure option named <code class="docutils literal notranslate">--with-computed-gotos</code>
on compilers that support it (notably: gcc, SunPro, icc), the bytecode
evaluation loop is compiled with a new dispatch mechanism which gives
speedups of up to 20%, depending on the system, the compiler, and
the benchmark.
(Contributed by Antoine Pitrou along with a number of other participants,
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4753">bpo-4753</a>).
</li>
<li>The decoding of UTF-8, UTF-16 and LATIN-1 is now two to four times
faster.
(Contributed by Antoine Pitrou and Amaury Forgeot d’Arc, <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4868">bpo-4868</a>.)
</li>
<li>The <a class="reference internal" href="../library/json.html#module-json" title="json: Encode and decode the JSON format."><code class="xref py py-mod docutils literal notranslate">json</code></a> module now has a C extension to substantially improve
its performance. In addition, the API was modified so that json works
only with <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a>, not with <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a>. That change makes the
module closely match the <a class="reference external" href="https://json.org/">JSON specification</a>
which is defined in terms of Unicode.
(Contributed by Bob Ippolito and converted to Py3.1 by Antoine Pitrou
and Benjamin Peterson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4136">bpo-4136</a>.)
</li>
<li>Unpickling now interns the attribute names of pickled objects. This saves
memory and allows pickles to be smaller.
(Contributed by Jake McGuire and Antoine Pitrou; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5084">bpo-5084</a>.)
</li>
</ul>
</section>
<section id="idle">
<h2>IDLE<a class="headerlink" href="#idle" title="Permalink to this headline">¶</a></h2>
<ul>
<li>IDLE’s format menu now provides an option to strip trailing whitespace
from a source file.
(Contributed by Roger D. Serwy; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5150">bpo-5150</a>.)
</li>
</ul>
</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>
Changes to Python’s build process and to the C API include:
<ul>
<li>Integers are now stored internally either in base <code class="docutils literal notranslate">2**15</code> or in base
<code class="docutils literal notranslate">2**30</code>, the base being determined at build time. Previously, they
were always stored in base <code class="docutils literal notranslate">2**15</code>. Using base <code class="docutils literal notranslate">2**30</code> gives
significant performance improvements on 64-bit machines, but
benchmark results on 32-bit machines have been mixed. Therefore,
the default is to use base <code class="docutils literal notranslate">2**30</code> on 64-bit machines and base <code class="docutils literal notranslate">2**15</code>
on 32-bit machines; on Unix, there’s a new configure option
<code class="docutils literal notranslate">--enable-big-digits</code> that can be used to override this default.
Apart from the performance improvements this change should be invisible to
end users, with one exception: for testing and debugging purposes there’s a
new <a class="reference internal" href="../library/sys.html#sys.int_info" title="sys.int_info"><code class="xref py py-attr docutils literal notranslate">sys.int_info</code></a> that provides information about the
internal format, giving the number of bits per digit and the size in bytes
of the C type used to store each digit:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import sys
&gt;&gt;&gt; sys.int_info
sys.int_info(bits_per_digit=30, sizeof_digit=4)
</pre></div>
</div>
(Contributed by Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4258">bpo-4258</a>.)
</li>
<li>The <a class="reference internal" href="../c-api/long.html#c.PyLong_AsUnsignedLongLong" title="PyLong_AsUnsignedLongLong"><code class="xref c c-func docutils literal notranslate">PyLong_AsUnsignedLongLong()</code></a> function now handles a negative
pylong by raising <a class="reference internal" href="../library/exceptions.html#OverflowError" title="OverflowError"><code class="xref py py-exc docutils literal notranslate">OverflowError</code></a> instead of <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
(Contributed by Mark Dickinson and Lisandro Dalcrin; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5175">bpo-5175</a>.)
</li>
<li>Deprecated <code class="xref c c-func docutils literal notranslate">PyNumber_Int()</code>. Use <a class="reference internal" href="../c-api/number.html#c.PyNumber_Long" title="PyNumber_Long"><code class="xref c c-func docutils literal notranslate">PyNumber_Long()</code></a> instead.
(Contributed by Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4910">bpo-4910</a>.)
</li>
<li>Added a new <a class="reference internal" href="../c-api/conversion.html#c.PyOS_string_to_double" title="PyOS_string_to_double"><code class="xref c c-func docutils literal notranslate">PyOS_string_to_double()</code></a> function to replace the
deprecated functions <code class="xref c c-func docutils literal notranslate">PyOS_ascii_strtod()</code> and <code class="xref c c-func docutils literal notranslate">PyOS_ascii_atof()</code>.
(Contributed by Mark Dickinson; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5914">bpo-5914</a>.)
</li>
<li>Added <a class="reference internal" href="../c-api/capsule.html#c.PyCapsule" title="PyCapsule"><code class="xref c c-type docutils literal notranslate">PyCapsule</code></a> as a replacement for the <code class="xref c c-type docutils literal notranslate">PyCObject</code> API.
The principal difference is that the new type has a well defined interface
for passing typing safety information and a less complicated signature
for calling a destructor. The old type had a problematic API and is now
deprecated.
(Contributed by Larry Hastings; <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5630">bpo-5630</a>.)
</li>
</ul>
</section>
<section id="porting-to-python-3-1">
<h2>Porting to Python 3.1<a class="headerlink" href="#porting-to-python-3-1" title="Permalink to this headline">¶</a></h2>
This section lists previously described changes and other bugfixes
that may require changes to your code:
<ul>
<li>The new floating point string representations can break existing doctests.
For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def e():
 &#39;&#39;&#39;Compute the base of natural logarithms.

&gt;&gt;&gt; e()
 2.7182818284590451

&#39;&#39;&#39;
 return sum(1/math.factorial(x) for x in reversed(range(30)))

doctest.testmod()

**********************************************************************
Failed example:
 e()
Expected:
 2.7182818284590451
Got:
 2.718281828459045
**********************************************************************
</pre></div>
</div>
</li>
<li>The automatic name remapping in the pickle module for protocol 2 or lower can
make Python 3.1 pickles unreadable in Python 3.0. One solution is to use
protocol 3. Another solution is to set the fix_imports option to <code class="docutils literal notranslate">False</code>.
See the discussion above for more details.</li>
</ul>
</section>
</section>

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 <ul>
<li><a class="reference internal" href="#">What’s New In Python 3.1</a><ul>
<li><a class="reference internal" href="#pep-372-ordered-dictionaries">PEP 372: Ordered Dictionaries</a></li>
<li><a class="reference internal" href="#pep-378-format-specifier-for-thousands-separator">PEP 378: Format Specifier for Thousands Separator</a></li>
<li><a class="reference internal" href="#other-language-changes">Other Language Changes</a></li>
<li><a class="reference internal" href="#new-improved-and-deprecated-modules">New, Improved, and Deprecated Modules</a></li>
<li><a class="reference internal" href="#optimizations">Optimizations</a></li>
<li><a class="reference internal" href="#idle">IDLE</a></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="#porting-to-python-3-1">Porting to Python 3.1</a></li>
</ul>
</li>
</ul>

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