: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:

.. list-table:: :widths: auto :header-rows: 1

• • feature
  • optional in
  • mandatory in
  • effect
• • .. data:: nested_scopes
  • 2.1.0b1
  • 2.2
  • :pep:227: Statically Nested Scopes
• • .. data:: generators
  • 2.2.0a1
  • 2.3
  • :pep:255: Simple Generators
• • .. data:: division
  • 2.2.0a2
  • 3.0
  • :pep:238: Changing the Division Operator
• • .. data:: absolute_import
  • 2.5.0a1
  • 3.0
  • :pep:328: Imports: Multi-Line and Absolute/Relative
• • .. data:: with_statement
  • 2.5.0a1
  • 2.6
  • :pep:343: The “with” Statement
• • .. data:: print_function
  • 2.6.0a2
  • 3.0
  • :pep:3105: Make print a function
• • .. data:: unicode_literals
  • 2.6.0a2
  • 3.0
  • :pep:3112: Bytes literals in Python 3000
• • .. data:: generator_stop
  • 3.5.0b1
  • 3.7
  • :pep:479: StopIteration handling inside generators
• • .. data:: annotations
  • 3.7.0b1
  • Never [1]_
  • :pep:563: Postponed evaluation of annotations, :pep:649: Deferred evaluation of annotations using descriptors

.. XXX Adding a new entry? Remember to update simple_stmts.rst, too.

.. _future-classes:

.. class:: _Feature

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.