: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.