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<H1><A NAME="SECTION0053400000000000000000">
3.34 <tt class="module">__future__</tt> --
         Future statement definitions</A>
</H1>

<P>
<A NAME="module-future"></A>

<P>
<tt class="module">__future__</tt> is a real module, and serves three purposes:

<P>

<UL>
<LI>To avoid confusing existing tools that analyze import statements
      and expect to find the modules they're importing.

<P>
</LI>
<LI>To ensure that future_statements run under releases prior to 2.1
      at least yield runtime exceptions (the import of
      <tt class="module">__future__</tt> will fail, because there was no module of
      that name prior to 2.1).

<P>
</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 programatically via importing
      <tt class="module">__future__</tt> and examining its contents.

<P>
</LI>
</UL>

<P>
Each statement in <span class="file">__future__.py</span> is of the form:

<P>
<div class="verbatim"><pre><TT>
 FeatureName = &#34;_Feature(&#34; <var>OptionalRelease</var> &#34;,&#34; <var>MandatoryRelease</var> &#34;,&#34;
                         <var>CompilerFlag</var> &#34;)&#34;
 </TT></pre></div>

<P>
where, normally, <var>OptionalRelease</var> is less than
<var>MandatoryRelease</var>, and both are 5-tuples of the same form as
<code>sys.version_info</code>:

<P>
<div class="verbatim"><pre>
    (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
     PY_MINOR_VERSION, # the 1; an int
     PY_MICRO_VERSION, # the 0; an int
     PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
     PY_RELEASE_SERIAL # the 3; an int
    )
</pre></div>

<P>
<var>OptionalRelease</var> records the first release in which the feature
was accepted.

<P>
In the case of a <var>MandatoryRelease</var> that has not yet occurred,
<var>MandatoryRelease</var> predicts the release in which the feature will
become part of the language.

<P>
Else <var>MandatoryRelease</var> 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>
<var>MandatoryRelease</var> may also be <code>None</code>, meaning that a planned
feature got dropped.

<P>
Instances of class <tt class="class">_Feature</tt> have two corresponding methods,
<tt class="method">getOptionalRelease()</tt> and <tt class="method">getMandatoryRelease()</tt>.

<P>
<var>CompilerFlag</var> is the (bitfield) flag that should be passed in the
fourth argument to the builtin function <tt class="function">compile()</tt> to enable
the feature in dynamically compiled code.  This flag is stored in the
<tt class="member">compiler_flag</tt> attribute on <tt class="class">_Future</tt> instances.

<P>
No feature description will ever be deleted from <tt class="module">__future__</tt>.

<P>

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