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__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 programatically 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 builtin 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__.
Release 2.4.5, documentation updated on 18 October 2006.
See About this document... for information on suggesting changes.
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