11. __future__
— Future statement definitions¶
Source code: :source:`Lib/__future__.py`
__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 future statements run under releases prior to
2.1 at least yield runtime exceptions (the import of
__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
__future__
and examining its contents.
Each statement in __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 _Feature
have two corresponding methods,
getOptionalRelease()
and getMandatoryRelease()
.
CompilerFlag is the (bitfield) flag that should be passed in the fourth
argument to the built-in function compile()
to enable the feature in
dynamically compiled code. This flag is stored in the compiler_flag
attribute on _Feature
instances.
No feature description will ever be deleted from __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 |
See also
- future
- How the compiler treats future imports.