Source code: 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
(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
Instances of class
_Feature have two corresponding methods,
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
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|
|generator_stop||3.5.0b1||3.7||PEP 479: StopIteration handling inside generators|
- Future statements
- How the compiler treats future imports.