1 :mod:`__future__` --- Future statement definitions
2 ==================================================
5 :synopsis: Future statement definitions
8 :mod:`__future__` is a real module, and serves three purposes:
10 * To avoid confusing existing tools that analyze import statements and expect to
11 find the modules they're importing.
13 * To ensure that :ref:`future statements <future>` run under releases prior to
14 2.1 at least yield runtime exceptions (the import of :mod:`__future__` will
15 fail, because there was no module of that name prior to 2.1).
17 * To document when incompatible changes were introduced, and when they will be
18 --- or were --- made mandatory. This is a form of executable documentation, and
19 can be inspected programmatically via importing :mod:`__future__` and examining
22 Each statement in :file:`__future__.py` is of the form::
24 FeatureName = _Feature(OptionalRelease, MandatoryRelease,
28 where, normally, *OptionalRelease* is less than *MandatoryRelease*, and both are
29 5-tuples of the same form as ``sys.version_info``::
31 (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
32 PY_MINOR_VERSION, # the 1; an int
33 PY_MICRO_VERSION, # the 0; an int
34 PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
35 PY_RELEASE_SERIAL # the 3; an int
38 *OptionalRelease* records the first release in which the feature was accepted.
40 In the case of a *MandatoryRelease* that has not yet occurred,
41 *MandatoryRelease* predicts the release in which the feature will become part of
44 Else *MandatoryRelease* records when the feature became part of the language; in
45 releases at or after that, modules no longer need a future statement to use the
46 feature in question, but may continue to use such imports.
48 *MandatoryRelease* may also be ``None``, meaning that a planned feature got
51 Instances of class :class:`_Feature` have two corresponding methods,
52 :meth:`getOptionalRelease` and :meth:`getMandatoryRelease`.
54 *CompilerFlag* is the (bitfield) flag that should be passed in the fourth
55 argument to the built-in function :func:`compile` to enable the feature in
56 dynamically compiled code. This flag is stored in the :attr:`compiler_flag`
57 attribute on :class:`_Feature` instances.
59 No feature description will ever be deleted from :mod:`__future__`. Since its
60 introduction in Python 2.1 the following features have found their way into the
61 language using this mechanism:
63 +------------------+-------------+--------------+---------------------------------------------+
64 | feature | optional in | mandatory in | effect |
65 +==================+=============+==============+=============================================+
66 | nested_scopes | 2.1.0b1 | 2.2 | :pep:`227`: |
67 | | | | *Statically Nested Scopes* |
68 +------------------+-------------+--------------+---------------------------------------------+
69 | generators | 2.2.0a1 | 2.3 | :pep:`255`: |
70 | | | | *Simple Generators* |
71 +------------------+-------------+--------------+---------------------------------------------+
72 | division | 2.2.0a2 | 3.0 | :pep:`238`: |
73 | | | | *Changing the Division Operator* |
74 +------------------+-------------+--------------+---------------------------------------------+
75 | absolute_import | 2.5.0a1 | 2.7 | :pep:`328`: |
76 | | | | *Imports: Multi-Line and Absolute/Relative* |
77 +------------------+-------------+--------------+---------------------------------------------+
78 | with_statement | 2.5.0a1 | 2.6 | :pep:`343`: |
79 | | | | *The "with" Statement* |
80 +------------------+-------------+--------------+---------------------------------------------+
81 | print_function | 2.6.0a2 | 3.0 | :pep:`3105`: |
82 | | | | *Make print a function* |
83 +------------------+-------------+--------------+---------------------------------------------+
84 | unicode_literals | 2.6.0a2 | 3.0 | :pep:`3112`: |
85 | | | | *Bytes literals in Python 3000* |
86 +------------------+-------------+--------------+---------------------------------------------+
91 How the compiler treats future imports.