Edit File: __future__.html
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Python Runtime Services</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-__future__"> <span id="future-future-statement-definitions"></span><h1>27.11. <a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><tt class="xref py py-mod docutils literal"><span class="pre">__future__</span></tt></a> — Future statement definitions<a class="headerlink" href="#module-__future__" title="Permalink to this headline">¶</a></h1> <p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/__future__.py">Lib/__future__.py</a></p> <hr class="docutils" /> <p><a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><tt class="xref py py-mod docutils literal"><span class="pre">__future__</span></tt></a> is a real module, and serves three purposes:</p> <ul class="simple"> <li>To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.</li> <li>To ensure that <a class="reference internal" href="../reference/simple_stmts.html#future"><em>future statements</em></a> run under releases prior to 2.1 at least yield runtime exceptions (the import of <a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><tt class="xref py py-mod docutils literal"><span class="pre">__future__</span></tt></a> will fail, because there was no module of that name prior to 2.1).</li> <li>To document when incompatible changes were introduced, and when they will be — or were — made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing <a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><tt class="xref py py-mod docutils literal"><span class="pre">__future__</span></tt></a> and examining its contents.</li> </ul> <p>Each statement in <tt class="file docutils literal"><span class="pre">__future__.py</span></tt> is of the form:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">FeatureName</span> <span class="o">=</span> <span class="n">_Feature</span><span class="p">(</span><span class="n">OptionalRelease</span><span class="p">,</span> <span class="n">MandatoryRelease</span><span class="p">,</span> <span class="n">CompilerFlag</span><span class="p">)</span> </pre></div> </div> <p>where, normally, <em>OptionalRelease</em> is less than <em>MandatoryRelease</em>, and both are 5-tuples of the same form as <tt class="docutils literal"><span class="pre">sys.version_info</span></tt>:</p> <div class="highlight-python"><div class="highlight"><pre><span class="p">(</span><span class="n">PY_MAJOR_VERSION</span><span class="p">,</span> <span class="c"># the 2 in 2.1.0a3; an int</span> <span class="n">PY_MINOR_VERSION</span><span class="p">,</span> <span class="c"># the 1; an int</span> <span class="n">PY_MICRO_VERSION</span><span class="p">,</span> <span class="c"># the 0; an int</span> <span class="n">PY_RELEASE_LEVEL</span><span class="p">,</span> <span class="c"># "alpha", "beta", "candidate" or "final"; string</span> <span class="n">PY_RELEASE_SERIAL</span> <span class="c"># the 3; an int</span> <span class="p">)</span> </pre></div> </div> <p><em>OptionalRelease</em> records the first release in which the feature was accepted.</p> <p>In the case of a <em>MandatoryRelease</em> that has not yet occurred, <em>MandatoryRelease</em> predicts the release in which the feature will become part of the language.</p> <p>Else <em>MandatoryRelease</em> records when the feature became part of the language; in releases at or after that, modules no longer need a future statement to use the feature in question, but may continue to use such imports.</p> <p><em>MandatoryRelease</em> may also be <tt class="docutils literal"><span class="pre">None</span></tt>, meaning that a planned feature got dropped.</p> <p>Instances of class <tt class="xref py py-class docutils literal"><span class="pre">_Feature</span></tt> have two corresponding methods, <tt class="xref py py-meth docutils literal"><span class="pre">getOptionalRelease()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">getMandatoryRelease()</span></tt>.</p> <p><em>CompilerFlag</em> is the (bitfield) flag that should be passed in the fourth argument to the built-in function <a class="reference internal" href="functions.html#compile" title="compile"><tt class="xref py py-func docutils literal"><span class="pre">compile()</span></tt></a> to enable the feature in dynamically compiled code. This flag is stored in the <tt class="xref py py-attr docutils literal"><span class="pre">compiler_flag</span></tt> attribute on <tt class="xref py py-class docutils literal"><span class="pre">_Feature</span></tt> instances.</p> <p>No feature description will ever be deleted from <a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><tt class="xref py py-mod docutils literal"><span class="pre">__future__</span></tt></a>. Since its introduction in Python 2.1 the following features have found their way into the language using this mechanism:</p> <table border="1" class="docutils"> <colgroup> <col width="20%" /> <col width="14%" /> <col width="16%" /> <col width="50%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">feature</th> <th class="head">optional in</th> <th class="head">mandatory in</th> <th class="head">effect</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td>nested_scopes</td> <td>2.1.0b1</td> <td>2.2</td> <td><span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0227"><strong>PEP 227</strong></a>: <em>Statically Nested Scopes</em></td> </tr> <tr class="row-odd"><td>generators</td> <td>2.2.0a1</td> <td>2.3</td> <td><span class="target" id="index-1"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0255"><strong>PEP 255</strong></a>: <em>Simple Generators</em></td> </tr> <tr class="row-even"><td>division</td> <td>2.2.0a2</td> <td>3.0</td> <td><span class="target" id="index-2"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0238"><strong>PEP 238</strong></a>: <em>Changing the Division Operator</em></td> </tr> <tr class="row-odd"><td>absolute_import</td> <td>2.5.0a1</td> <td>3.0</td> <td><span class="target" id="index-3"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0328"><strong>PEP 328</strong></a>: <em>Imports: Multi-Line and Absolute/Relative</em></td> </tr> <tr class="row-even"><td>with_statement</td> <td>2.5.0a1</td> <td>2.6</td> <td><span class="target" id="index-4"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a>: <em>The “with” Statement</em></td> </tr> <tr class="row-odd"><td>print_function</td> <td>2.6.0a2</td> <td>3.0</td> <td><span class="target" id="index-5"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-3105"><strong>PEP 3105</strong></a>: <em>Make print a function</em></td> </tr> <tr class="row-even"><td>unicode_literals</td> <td>2.6.0a2</td> <td>3.0</td> <td><span class="target" id="index-6"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-3112"><strong>PEP 3112</strong></a>: <em>Bytes literals in Python 3000</em></td> </tr> </tbody> </table> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference internal" href="../reference/simple_stmts.html#future"><em>Future statements</em></a></dt> <dd>How the compiler treats future imports.</dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="traceback.html" title="previous chapter">27.10. <tt class="docutils literal"><span class="pre">traceback</span></tt> — Print or retrieve a stack traceback</a></p> <h4>Next topic</h4> <p class="topless"><a href="gc.html" title="next chapter">27.12. <tt class="docutils literal"><span class="pre">gc</span></tt> — Garbage Collector interface</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/__future__.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="gc.html" title="27.12. gc — Garbage Collector interface" >next</a> |</li> <li class="right" > <a href="traceback.html" title="27.10. traceback — Print or retrieve a stack traceback" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" >The Python Standard Library</a> »</li> <li><a href="python.html" >27. 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