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+:mod:`__future__` --- Future statement definitions
+==================================================
+
+.. module:: __future__
+   :synopsis: Future statement definitions
+
+**Source code:** :source:`Lib/__future__.py`
+
+--------------
+
+:mod:`__future__` is a real module, and serves three purposes:
+
+* To avoid confusing existing tools that analyze import statements and expect to
+  find the modules they're importing.
+
+* To ensure that :ref:`future statements <future>` run under releases prior to
+  2.1 at least yield runtime exceptions (the import of :mod:`__future__` will
+  fail, because there was no module of that name prior to 2.1).
+
+* 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 :mod:`__future__` and examining
+  its contents.
+
+Each statement in :file:`__future__.py` is of the form::
+
+   FeatureName = _Feature(OptionalRelease, MandatoryRelease,
+                          CompilerFlag)
+
+
+where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
+5-tuples of the same form as :data:`sys.version_info`::
+
+   (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
+   )
+
+*OptionalRelease* records the first release in which the feature was accepted.
+
+In the case of a *MandatoryRelease* that has not yet occurred,
+*MandatoryRelease* predicts the release in which the feature will become part of
+the language.
+
+Else *MandatoryRelease* 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.
+
+*MandatoryRelease* may also be ``None``, meaning that a planned feature got
+dropped.
+
+Instances of class :class:`_Feature` have two corresponding methods,
+:meth:`getOptionalRelease` and :meth:`getMandatoryRelease`.
+
+*CompilerFlag* is the (bitfield) flag that should be passed in the fourth
+argument to the built-in function :func:`compile` to enable the feature in
+dynamically compiled code.  This flag is stored in the :attr:`compiler_flag`
+attribute on :class:`_Feature` instances.
+
+No feature description will ever be deleted from :mod:`__future__`. Since its
+introduction in Python 2.1 the following features have found their way into the
+language using this mechanism:
+
++------------------+-------------+--------------+---------------------------------------------+
+| feature          | optional in | mandatory in | effect                                      |
++==================+=============+==============+=============================================+
+| nested_scopes    | 2.1.0b1     | 2.2          | :pep:`227`:                                 |
+|                  |             |              | *Statically Nested Scopes*                  |
++------------------+-------------+--------------+---------------------------------------------+
+| generators       | 2.2.0a1     | 2.3          | :pep:`255`:                                 |
+|                  |             |              | *Simple Generators*                         |
++------------------+-------------+--------------+---------------------------------------------+
+| division         | 2.2.0a2     | 3.0          | :pep:`238`:                                 |
+|                  |             |              | *Changing the Division Operator*            |
++------------------+-------------+--------------+---------------------------------------------+
+| absolute_import  | 2.5.0a1     | 3.0          | :pep:`328`:                                 |
+|                  |             |              | *Imports: Multi-Line and Absolute/Relative* |
++------------------+-------------+--------------+---------------------------------------------+
+| with_statement   | 2.5.0a1     | 2.6          | :pep:`343`:                                 |
+|                  |             |              | *The "with" Statement*                      |
++------------------+-------------+--------------+---------------------------------------------+
+| print_function   | 2.6.0a2     | 3.0          | :pep:`3105`:                                |
+|                  |             |              | *Make print a function*                     |
++------------------+-------------+--------------+---------------------------------------------+
+| unicode_literals | 2.6.0a2     | 3.0          | :pep:`3112`:                                |
+|                  |             |              | *Bytes literals in Python 3000*             |
++------------------+-------------+--------------+---------------------------------------------+
+| generator_stop   | 3.5.0b1     | 3.7          | :pep:`479`:                                 |
+|                  |             |              | *StopIteration handling inside generators*  |
++------------------+-------------+--------------+---------------------------------------------+
+| annotations      | 3.7.0b1     | 4.0          | :pep:`563`:                                 |
+|                  |             |              | *Postponed evaluation of annotations*       |
++------------------+-------------+--------------+---------------------------------------------+
+
+.. XXX Adding a new entry?  Remember to update simple_stmts.rst, too.
+
+
+.. seealso::
+
+   :ref:`future`
+      How the compiler treats future imports.