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<title>3.34 __future__ -- Future statement definitions
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<H1><A NAME=
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3.34 <tt class=
"module">__future__
</tt> --
Future statement definitions
</A>
<A NAME=
"module-future"></A>
<tt class=
"module">__future__
</tt> is a real module, and serves three purposes:
<LI>To avoid confusing existing tools that analyze import statements
and expect to find the modules they're importing.
<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
<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.
Each statement in
<span class=
"file">__future__.py
</span> is of the form:
<div class=
"verbatim"><pre><TT>
FeatureName =
"_Feature(
" <var>OptionalRelease
</var> ",
" <var>MandatoryRelease
</var> ",
"
<var>CompilerFlag
</var> ")
"
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>:
<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
<var>OptionalRelease
</var> records the first release in which the feature
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.
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
<var>MandatoryRelease
</var> may also be
<code>None
</code>, meaning that a planned
Instances of class
<tt class=
"class">_Feature
</tt> have two corresponding methods,
<tt class=
"method">getOptionalRelease()
</tt> and
<tt class=
"method">getMandatoryRelease()
</tt>.
<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.
No feature description will ever be deleted from
<tt class=
"module">__future__
</tt>.
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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