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hauntsaninja merged 1 commit into from
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gh-110893: Improve the documentation for __future__ module #114642

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98 changes: 56 additions & 42 deletions Doc/library/__future__.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -8,20 +8,68 @@

--------------

:mod:`__future__` is a real module, and serves three purposes:
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 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.

* 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
Expand Down Expand Up @@ -65,43 +113,6 @@
dynamically compiled code. This flag is stored in the :attr:`_Feature.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 | TBD [1]_ | :pep:`563`: |
| | | | *Postponed evaluation of annotations* |
+------------------+-------------+--------------+---------------------------------------------+

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

.. [1]
``from __future__ import annotations`` was previously scheduled to
become mandatory in Python 3.10, but the Python Steering Council
Expand All @@ -115,3 +126,6 @@ language using this mechanism:

:ref:`future`
How the compiler treats future imports.

:pep:`236` - Back to the __future__
The original proposal for the __future__ mechanism.