<!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta charset="utf-8" /> <title>__future__ — Future statement definitions — Python 3.7.4 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.7.4 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="gc — Garbage Collector interface" href="gc.html" /> <link rel="prev" title="traceback — Print or retrieve a stack traceback" href="traceback.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/__future__.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> <script type="text/javascript" src="../_static/switchers.js"></script> <style> @media only screen { table.full-width-table { width: 100%; } } </style> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">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="gc — Garbage Collector interface" accesskey="N">next</a> |</li> <li class="right" > <a href="traceback.html" title="traceback — Print or retrieve a stack traceback" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <span class="language_switcher_placeholder">en</span> <span class="version_switcher_placeholder">3.7.4</span> <a href="../index.html">Documentation </a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="python.html" accesskey="U">Python Runtime Services</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" 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> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-__future__"> <span id="future-future-statement-definitions"></span><h1><a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></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="https://github.com/python/cpython/tree/3.7/Lib/__future__.py">Lib/__future__.py</a></p> <hr class="docutils" /> <p><a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a> is a real module, and serves three purposes:</p> <ul class="simple"> <li><p>To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.</p></li> <li><p>To ensure that <a class="reference internal" href="../reference/simple_stmts.html#future"><span class="std std-ref">future statements</span></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"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a> will fail, because there was no module of that name prior to 2.1).</p></li> <li><p>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"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a> and examining its contents.</p></li> </ul> <p>Each statement in <code class="file docutils literal notranslate"><span class="pre">__future__.py</span></code> is of the form:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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 <a class="reference internal" href="sys.html#sys.version_info" title="sys.version_info"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.version_info</span></code></a>:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">PY_MAJOR_VERSION</span><span class="p">,</span> <span class="c1"># the 2 in 2.1.0a3; an int</span> <span class="n">PY_MINOR_VERSION</span><span class="p">,</span> <span class="c1"># the 1; an int</span> <span class="n">PY_MICRO_VERSION</span><span class="p">,</span> <span class="c1"># the 0; an int</span> <span class="n">PY_RELEASE_LEVEL</span><span class="p">,</span> <span class="c1"># "alpha", "beta", "candidate" or "final"; string</span> <span class="n">PY_RELEASE_SERIAL</span> <span class="c1"># 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 <code class="docutils literal notranslate"><span class="pre">None</span></code>, meaning that a planned feature got dropped.</p> <p>Instances of class <code class="xref py py-class docutils literal notranslate"><span class="pre">_Feature</span></code> have two corresponding methods, <code class="xref py py-meth docutils literal notranslate"><span class="pre">getOptionalRelease()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">getMandatoryRelease()</span></code>.</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"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> to enable the feature in dynamically compiled code. This flag is stored in the <code class="xref py py-attr docutils literal notranslate"><span class="pre">compiler_flag</span></code> attribute on <code class="xref py py-class docutils literal notranslate"><span class="pre">_Feature</span></code> instances.</p> <p>No feature description will ever be deleted from <a class="reference internal" href="#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a>. Since its introduction in Python 2.1 the following features have found their way into the language using this mechanism:</p> <table class="docutils align-center"> <colgroup> <col style="width: 20%" /> <col style="width: 14%" /> <col style="width: 16%" /> <col style="width: 50%" /> </colgroup> <thead> <tr class="row-odd"><th class="head"><p>feature</p></th> <th class="head"><p>optional in</p></th> <th class="head"><p>mandatory in</p></th> <th class="head"><p>effect</p></th> </tr> </thead> <tbody> <tr class="row-even"><td><p>nested_scopes</p></td> <td><p>2.1.0b1</p></td> <td><p>2.2</p></td> <td><p><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0227"><strong>PEP 227</strong></a>: <em>Statically Nested Scopes</em></p></td> </tr> <tr class="row-odd"><td><p>generators</p></td> <td><p>2.2.0a1</p></td> <td><p>2.3</p></td> <td><p><span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0255"><strong>PEP 255</strong></a>: <em>Simple Generators</em></p></td> </tr> <tr class="row-even"><td><p>division</p></td> <td><p>2.2.0a2</p></td> <td><p>3.0</p></td> <td><p><span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0238"><strong>PEP 238</strong></a>: <em>Changing the Division Operator</em></p></td> </tr> <tr class="row-odd"><td><p>absolute_import</p></td> <td><p>2.5.0a1</p></td> <td><p>3.0</p></td> <td><p><span class="target" id="index-3"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0328"><strong>PEP 328</strong></a>: <em>Imports: Multi-Line and Absolute/Relative</em></p></td> </tr> <tr class="row-even"><td><p>with_statement</p></td> <td><p>2.5.0a1</p></td> <td><p>2.6</p></td> <td><p><span class="target" id="index-4"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a>: <em>The “with” Statement</em></p></td> </tr> <tr class="row-odd"><td><p>print_function</p></td> <td><p>2.6.0a2</p></td> <td><p>3.0</p></td> <td><p><span class="target" id="index-5"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3105"><strong>PEP 3105</strong></a>: <em>Make print a function</em></p></td> </tr> <tr class="row-even"><td><p>unicode_literals</p></td> <td><p>2.6.0a2</p></td> <td><p>3.0</p></td> <td><p><span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3112"><strong>PEP 3112</strong></a>: <em>Bytes literals in Python 3000</em></p></td> </tr> <tr class="row-odd"><td><p>generator_stop</p></td> <td><p>3.5.0b1</p></td> <td><p>3.7</p></td> <td><p><span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0479"><strong>PEP 479</strong></a>: <em>StopIteration handling inside generators</em></p></td> </tr> <tr class="row-even"><td><p>annotations</p></td> <td><p>3.7.0b1</p></td> <td><p>4.0</p></td> <td><p><span class="target" id="index-8"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0563"><strong>PEP 563</strong></a>: <em>Postponed evaluation of annotations</em></p></td> </tr> </tbody> </table> <div class="admonition seealso"> <p class="admonition-title">See also</p> <dl class="simple"> <dt><a class="reference internal" href="../reference/simple_stmts.html#future"><span class="std std-ref">Future statements</span></a></dt><dd><p>How the compiler treats future imports.</p> </dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="traceback.html" title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">traceback</span></code> — Print or retrieve a stack traceback</a></p> <h4>Next topic</h4> <p class="topless"><a href="gc.html" title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code> — Garbage Collector interface</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.7/Doc/library/__future__.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <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="gc — Garbage Collector interface" >next</a> |</li> <li class="right" > <a href="traceback.html" title="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="https://www.python.org/">Python</a> »</li> <li> <span class="language_switcher_placeholder">en</span> <span class="version_switcher_placeholder">3.7.4</span> <a href="../index.html">Documentation </a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="python.html" >Python Runtime Services</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" 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> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 2001-2019, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Jul 13, 2019. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 2.0.1. </div> </body> </html>