mirror of https://github.com/python/cpython
132 lines
6.7 KiB
ReStructuredText
132 lines
6.7 KiB
ReStructuredText
:mod:`!__future__` --- Future statement definitions
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===================================================
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.. module:: __future__
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:synopsis: Future statement definitions
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**Source code:** :source:`Lib/__future__.py`
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--------------
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Imports of the form ``from __future__ import feature`` are called
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:ref:`future statements <future>`. These are special-cased by the Python compiler
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to allow the use of new Python features in modules containing the future statement
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before the release in which the feature becomes standard.
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While these future statements are given additional special meaning by the
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Python compiler, they are still executed like any other import statement and
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the :mod:`__future__` exists and is handled by the import system the same way
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any other Python module would be. This design serves three purposes:
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* To avoid confusing existing tools that analyze import statements and expect to
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find the modules they're importing.
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* To document when incompatible changes were introduced, and when they will be
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--- or were --- made mandatory. This is a form of executable documentation, and
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can be inspected programmatically via importing :mod:`__future__` and examining
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its contents.
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* To ensure that :ref:`future statements <future>` run under releases prior to
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Python 2.1 at least yield runtime exceptions (the import of :mod:`__future__`
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will fail, because there was no module of that name prior to 2.1).
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Module Contents
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---------------
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No feature description will ever be deleted from :mod:`__future__`. Since its
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introduction in Python 2.1 the following features have found their way into the
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language using this mechanism:
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+------------------+-------------+--------------+---------------------------------------------+
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| feature | optional in | mandatory in | effect |
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+==================+=============+==============+=============================================+
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| nested_scopes | 2.1.0b1 | 2.2 | :pep:`227`: |
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| | | | *Statically Nested Scopes* |
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+------------------+-------------+--------------+---------------------------------------------+
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| generators | 2.2.0a1 | 2.3 | :pep:`255`: |
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| | | | *Simple Generators* |
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+------------------+-------------+--------------+---------------------------------------------+
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| division | 2.2.0a2 | 3.0 | :pep:`238`: |
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| | | | *Changing the Division Operator* |
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+------------------+-------------+--------------+---------------------------------------------+
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| absolute_import | 2.5.0a1 | 3.0 | :pep:`328`: |
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| | | | *Imports: Multi-Line and Absolute/Relative* |
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+------------------+-------------+--------------+---------------------------------------------+
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| with_statement | 2.5.0a1 | 2.6 | :pep:`343`: |
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| | | | *The "with" Statement* |
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+------------------+-------------+--------------+---------------------------------------------+
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| print_function | 2.6.0a2 | 3.0 | :pep:`3105`: |
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| | | | *Make print a function* |
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+------------------+-------------+--------------+---------------------------------------------+
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| unicode_literals | 2.6.0a2 | 3.0 | :pep:`3112`: |
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| | | | *Bytes literals in Python 3000* |
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+------------------+-------------+--------------+---------------------------------------------+
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| generator_stop | 3.5.0b1 | 3.7 | :pep:`479`: |
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| | | | *StopIteration handling inside generators* |
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+------------------+-------------+--------------+---------------------------------------------+
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| annotations | 3.7.0b1 | Never [1]_ | :pep:`563`: |
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| | | | *Postponed evaluation of annotations*, |
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| | | | :pep:`649`: *Deferred evaluation of |
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| | | | annotations using descriptors* |
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+------------------+-------------+--------------+---------------------------------------------+
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.. XXX Adding a new entry? Remember to update simple_stmts.rst, too.
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.. _future-classes:
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.. class:: _Feature
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Each statement in :file:`__future__.py` is of the form::
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FeatureName = _Feature(OptionalRelease, MandatoryRelease,
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CompilerFlag)
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where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
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5-tuples of the same form as :data:`sys.version_info`::
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(PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
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PY_MINOR_VERSION, # the 1; an int
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PY_MICRO_VERSION, # the 0; an int
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PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
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PY_RELEASE_SERIAL # the 3; an int
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)
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.. method:: _Feature.getOptionalRelease()
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*OptionalRelease* records the first release in which the feature was accepted.
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.. method:: _Feature.getMandatoryRelease()
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In the case of a *MandatoryRelease* that has not yet occurred,
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*MandatoryRelease* predicts the release in which the feature will become part of
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the language.
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Else *MandatoryRelease* records when the feature became part of the language; in
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releases at or after that, modules no longer need a future statement to use the
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feature in question, but may continue to use such imports.
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*MandatoryRelease* may also be ``None``, meaning that a planned feature got
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dropped or that it is not yet decided.
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.. attribute:: _Feature.compiler_flag
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*CompilerFlag* is the (bitfield) flag that should be passed in the fourth
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argument to the built-in function :func:`compile` to enable the feature in
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dynamically compiled code. This flag is stored in the :attr:`_Feature.compiler_flag`
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attribute on :class:`_Feature` instances.
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.. [1]
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``from __future__ import annotations`` was previously scheduled to
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become mandatory in Python 3.10, but the change was delayed and ultimately
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canceled. This feature will eventually be deprecated and removed. See
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:pep:`649` and :pep:`749`.
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.. seealso::
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:ref:`future`
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How the compiler treats future imports.
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:pep:`236` - Back to the __future__
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The original proposal for the __future__ mechanism.
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