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  &lt;div class=&quot;section&quot; id=&quot;module-__future__&quot;&gt;
&lt;span id=&quot;future-future-statement-definitions&quot;&gt;&lt;/span&gt;&lt;h1&gt;27.11. &lt;a class=&quot;reference internal&quot; href=&quot;#module-__future__&quot; title=&quot;__future__: Future statement definitions&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; &amp;#8212; Future statement definitions&lt;a class=&quot;headerlink&quot; href=&quot;#module-__future__&quot; title=&quot;Permalink to this headline&quot;&gt;¶&lt;/a&gt;&lt;/h1&gt;
&lt;p&gt;&lt;strong&gt;Source code:&lt;/strong&gt; &lt;a class=&quot;reference external&quot; href=&quot;http://hg.python.org/cpython/file/2.7/Lib/__future__.py&quot;&gt;Lib/__future__.py&lt;/a&gt;&lt;/p&gt;
&lt;hr class=&quot;docutils&quot; /&gt;
&lt;p&gt;&lt;a class=&quot;reference internal&quot; href=&quot;#module-__future__&quot; title=&quot;__future__: Future statement definitions&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; is a real module, and serves three purposes:&lt;/p&gt;
&lt;ul class=&quot;simple&quot;&gt;
&lt;li&gt;To avoid confusing existing tools that analyze import statements and expect to
find the modules they&amp;#8217;re importing.&lt;/li&gt;
&lt;li&gt;To ensure that &lt;a class=&quot;reference internal&quot; href=&quot;../reference/simple_stmts.html#future&quot;&gt;&lt;em&gt;future statements&lt;/em&gt;&lt;/a&gt; run under releases prior to
2.1 at least yield runtime exceptions (the import of &lt;a class=&quot;reference internal&quot; href=&quot;#module-__future__&quot; title=&quot;__future__: Future statement definitions&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; will
fail, because there was no module of that name prior to 2.1).&lt;/li&gt;
&lt;li&gt;To document when incompatible changes were introduced, and when they will be
&amp;#8212; or were &amp;#8212; made mandatory.  This is a form of executable documentation, and
can be inspected programmatically via importing &lt;a class=&quot;reference internal&quot; href=&quot;#module-__future__&quot; title=&quot;__future__: Future statement definitions&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; and examining
its contents.&lt;/li&gt;
&lt;/ul&gt;
&lt;p&gt;Each statement in &lt;tt class=&quot;file docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__.py&lt;/span&gt;&lt;/tt&gt; is of the form:&lt;/p&gt;
&lt;div class=&quot;highlight-python&quot;&gt;&lt;div class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;span class=&quot;n&quot;&gt;FeatureName&lt;/span&gt; &lt;span class=&quot;o&quot;&gt;=&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;_Feature&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;OptionalRelease&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;n&quot;&gt;MandatoryRelease&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt;
                       &lt;span class=&quot;n&quot;&gt;CompilerFlag&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;)&lt;/span&gt;
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;p&gt;where, normally, &lt;em&gt;OptionalRelease&lt;/em&gt; is less than &lt;em&gt;MandatoryRelease&lt;/em&gt;, and both are
5-tuples of the same form as &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;sys.version_info&lt;/span&gt;&lt;/tt&gt;:&lt;/p&gt;
&lt;div class=&quot;highlight-python&quot;&gt;&lt;div class=&quot;highlight&quot;&gt;&lt;pre&gt;&lt;span class=&quot;p&quot;&gt;(&lt;/span&gt;&lt;span class=&quot;n&quot;&gt;PY_MAJOR_VERSION&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;c&quot;&gt;# the 2 in 2.1.0a3; an int&lt;/span&gt;
 &lt;span class=&quot;n&quot;&gt;PY_MINOR_VERSION&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;c&quot;&gt;# the 1; an int&lt;/span&gt;
 &lt;span class=&quot;n&quot;&gt;PY_MICRO_VERSION&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;c&quot;&gt;# the 0; an int&lt;/span&gt;
 &lt;span class=&quot;n&quot;&gt;PY_RELEASE_LEVEL&lt;/span&gt;&lt;span class=&quot;p&quot;&gt;,&lt;/span&gt; &lt;span class=&quot;c&quot;&gt;# &amp;quot;alpha&amp;quot;, &amp;quot;beta&amp;quot;, &amp;quot;candidate&amp;quot; or &amp;quot;final&amp;quot;; string&lt;/span&gt;
 &lt;span class=&quot;n&quot;&gt;PY_RELEASE_SERIAL&lt;/span&gt; &lt;span class=&quot;c&quot;&gt;# the 3; an int&lt;/span&gt;
&lt;span class=&quot;p&quot;&gt;)&lt;/span&gt;
&lt;/pre&gt;&lt;/div&gt;
&lt;/div&gt;
&lt;p&gt;&lt;em&gt;OptionalRelease&lt;/em&gt; records the first release in which the feature was accepted.&lt;/p&gt;
&lt;p&gt;In the case of a &lt;em&gt;MandatoryRelease&lt;/em&gt; that has not yet occurred,
&lt;em&gt;MandatoryRelease&lt;/em&gt; predicts the release in which the feature will become part of
the language.&lt;/p&gt;
&lt;p&gt;Else &lt;em&gt;MandatoryRelease&lt;/em&gt; 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.&lt;/p&gt;
&lt;p&gt;&lt;em&gt;MandatoryRelease&lt;/em&gt; may also be &lt;tt class=&quot;docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;None&lt;/span&gt;&lt;/tt&gt;, meaning that a planned feature got
dropped.&lt;/p&gt;
&lt;p&gt;Instances of class &lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;_Feature&lt;/span&gt;&lt;/tt&gt; have two corresponding methods,
&lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;getOptionalRelease()&lt;/span&gt;&lt;/tt&gt; and &lt;tt class=&quot;xref py py-meth docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;getMandatoryRelease()&lt;/span&gt;&lt;/tt&gt;.&lt;/p&gt;
&lt;p&gt;&lt;em&gt;CompilerFlag&lt;/em&gt; is the (bitfield) flag that should be passed in the fourth
argument to the built-in function &lt;a class=&quot;reference internal&quot; href=&quot;functions.html#compile&quot; title=&quot;compile&quot;&gt;&lt;tt class=&quot;xref py py-func docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;compile()&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt; to enable the feature in
dynamically compiled code.  This flag is stored in the &lt;tt class=&quot;xref py py-attr docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;compiler_flag&lt;/span&gt;&lt;/tt&gt;
attribute on &lt;tt class=&quot;xref py py-class docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;_Feature&lt;/span&gt;&lt;/tt&gt; instances.&lt;/p&gt;
&lt;p&gt;No feature description will ever be deleted from &lt;a class=&quot;reference internal&quot; href=&quot;#module-__future__&quot; title=&quot;__future__: Future statement definitions&quot;&gt;&lt;tt class=&quot;xref py py-mod docutils literal&quot;&gt;&lt;span class=&quot;pre&quot;&gt;__future__&lt;/span&gt;&lt;/tt&gt;&lt;/a&gt;. Since its
introduction in Python 2.1 the following features have found their way into the
language using this mechanism:&lt;/p&gt;
&lt;table border=&quot;1&quot; class=&quot;docutils&quot;&gt;
&lt;colgroup&gt;
&lt;col width=&quot;20%&quot; /&gt;
&lt;col width=&quot;14%&quot; /&gt;
&lt;col width=&quot;16%&quot; /&gt;
&lt;col width=&quot;50%&quot; /&gt;
&lt;/colgroup&gt;
&lt;thead valign=&quot;bottom&quot;&gt;
&lt;tr class=&quot;row-odd&quot;&gt;&lt;th class=&quot;head&quot;&gt;feature&lt;/th&gt;
&lt;th class=&quot;head&quot;&gt;optional in&lt;/th&gt;
&lt;th class=&quot;head&quot;&gt;mandatory in&lt;/th&gt;
&lt;th class=&quot;head&quot;&gt;effect&lt;/th&gt;
&lt;/tr&gt;
&lt;/thead&gt;
&lt;tbody valign=&quot;top&quot;&gt;
&lt;tr class=&quot;row-even&quot;&gt;&lt;td&gt;nested_scopes&lt;/td&gt;
&lt;td&gt;2.1.0b1&lt;/td&gt;
&lt;td&gt;2.2&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-0&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-0227&quot;&gt;&lt;strong&gt;PEP 227&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Statically Nested Scopes&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-odd&quot;&gt;&lt;td&gt;generators&lt;/td&gt;
&lt;td&gt;2.2.0a1&lt;/td&gt;
&lt;td&gt;2.3&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-1&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-0255&quot;&gt;&lt;strong&gt;PEP 255&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Simple Generators&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-even&quot;&gt;&lt;td&gt;division&lt;/td&gt;
&lt;td&gt;2.2.0a2&lt;/td&gt;
&lt;td&gt;3.0&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-2&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-0238&quot;&gt;&lt;strong&gt;PEP 238&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Changing the Division Operator&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-odd&quot;&gt;&lt;td&gt;absolute_import&lt;/td&gt;
&lt;td&gt;2.5.0a1&lt;/td&gt;
&lt;td&gt;3.0&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-3&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-0328&quot;&gt;&lt;strong&gt;PEP 328&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Imports: Multi-Line and Absolute/Relative&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-even&quot;&gt;&lt;td&gt;with_statement&lt;/td&gt;
&lt;td&gt;2.5.0a1&lt;/td&gt;
&lt;td&gt;2.6&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-4&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-0343&quot;&gt;&lt;strong&gt;PEP 343&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;The &amp;#8220;with&amp;#8221; Statement&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-odd&quot;&gt;&lt;td&gt;print_function&lt;/td&gt;
&lt;td&gt;2.6.0a2&lt;/td&gt;
&lt;td&gt;3.0&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-5&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-3105&quot;&gt;&lt;strong&gt;PEP 3105&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Make print a function&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr class=&quot;row-even&quot;&gt;&lt;td&gt;unicode_literals&lt;/td&gt;
&lt;td&gt;2.6.0a2&lt;/td&gt;
&lt;td&gt;3.0&lt;/td&gt;
&lt;td&gt;&lt;span class=&quot;target&quot; id=&quot;index-6&quot;&gt;&lt;/span&gt;&lt;a class=&quot;pep reference external&quot; href=&quot;http://www.python.org/dev/peps/pep-3112&quot;&gt;&lt;strong&gt;PEP 3112&lt;/strong&gt;&lt;/a&gt;:
&lt;em&gt;Bytes literals in Python 3000&lt;/em&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;/tbody&gt;
&lt;/table&gt;
&lt;div class=&quot;admonition-see-also admonition seealso&quot;&gt;
&lt;p class=&quot;first admonition-title&quot;&gt;See also&lt;/p&gt;
&lt;dl class=&quot;last docutils&quot;&gt;
&lt;dt&gt;&lt;a class=&quot;reference internal&quot; href=&quot;../reference/simple_stmts.html#future&quot;&gt;&lt;em&gt;Future statements&lt;/em&gt;&lt;/a&gt;&lt;/dt&gt;
&lt;dd&gt;How the compiler treats future imports.&lt;/dd&gt;
&lt;/dl&gt;
&lt;/div&gt;
&lt;/div&gt;


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