| :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 | TBD [1]_ | :pep:`563`: | |
| | | | | *Postponed evaluation of annotations* | |
| +------------------+-------------+--------------+---------------------------------------------+ |
|
|
| .. 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 Python Steering Council |
| twice decided to delay the change |
| (`announcement for Python 3.10 <https://mail.python.org/archives/list/python-dev@python.org/message/CLVXXPQ2T2LQ5MP2Y53VVQFCXYWQJHKZ/>`__; |
| `announcement for Python 3.11 <https://mail.python.org/archives/list/python-dev@python.org/message/VIZEBX5EYMSYIJNDBF6DMUMZOCWHARSO/>`__). |
| No final decision has been made yet. See also :pep:`563` and :pep:`649`. |
|
|
|
|
| .. seealso:: |
|
|
| :ref:`future` |
| How the compiler treats future imports. |
|
|
| :pep:`236` - Back to the __future__ |
| The original proposal for the __future__ mechanism. |
|
|
