Edit File: __future__.txt
: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 ``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* | +------------------+-------------+--------------+---------------------------------------------+ .. seealso:: :ref:`future` How the compiler treats future imports.