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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.

	: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.