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<div class="section" id="module-__future__">
<span id="future-future-statement-definitions"></span><h1>30.11. <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="本標題的永久連結">¶</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>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"><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).</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"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a> and examining
its contents.</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"># &quot;alpha&quot;, &quot;beta&quot;, &quot;candidate&quot; or &quot;final&quot;; 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 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="https://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="https://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="https://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="https://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="https://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="https://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="https://www.python.org/dev/peps/pep-3112"><strong>PEP 3112</strong></a>:
<em>Bytes literals in Python 3000</em></td>
</tr>
<tr class="row-odd"><td>generator_stop</td>
<td>3.5.0b1</td>
<td>3.7</td>
<td><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></td>
</tr>
<tr class="row-even"><td>annotations</td>
<td>3.7.0b1</td>
<td>4.0</td>
<td><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></td>
</tr>
</tbody>
</table>
<div class="admonition seealso">
<p class="first admonition-title">也參考</p>
<dl class="last docutils">
<dt><a class="reference internal" href="../reference/simple_stmts.html#future"><span class="std std-ref">Future statements</span></a></dt>
<dd>How the compiler treats future imports.</dd>
</dl>
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