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:mod:`!__future__` --- Future statement definitions

.. module:: __future__
   :synopsis: Future statement definitions

Source code: :source:`Lib/__future__.py`

Imports of the form from __future__ import feature are called :ref:`future statements <future>`. These are special-cased by the Python compiler to allow the use of new Python features in modules containing the future statement before the release in which the feature becomes standard.

While these future statements are given additional special meaning by the Python compiler, they are still executed like any other import statement and the :mod:`__future__` exists and is handled by the import system the same way any other Python module would be. This design serves three purposes:

  • To avoid confusing existing tools that analyze import statements and expect to find the modules they're importing.
  • 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.
  • To ensure that :ref:`future statements <future>` run under releases prior to Python 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).

Module Contents

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 Never [1] PEP 563: Postponed evaluation of annotations, PEP 649: Deferred evaluation of annotations using descriptors

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
)

.. method:: _Feature.getOptionalRelease()

   *OptionalRelease* records the first release in which the feature was accepted.


.. method:: _Feature.getMandatoryRelease()

   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 or that it is not yet decided.


.. attribute:: _Feature.compiler_flag

   *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:`_Feature.compiler_flag`
   attribute on :class:`_Feature` instances.
[1]from __future__ import annotations was previously scheduled to become mandatory in Python 3.10, but the change was delayed and ultimately canceled. This feature will eventually be deprecated and removed. See PEP 649 and PEP 749.
 
.. seealso::

   :ref:`future`
      How the compiler treats future imports.

   :pep:`236` - Back to the __future__
      The original proposal for the __future__ mechanism.