28.10.__future__ — Future statement definitions¶

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 tofind the modules they’re importing.
• To ensure thatfuture statements run under releases prior to2.1 at least yield runtime exceptions (the import of__future__ willfail, 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, andcan be inspected programmatically via importing__future__ and examiningits contents.

Each statement in__future__.py is of the form:

FeatureName=_Feature(OptionalRelease,MandatoryRelease,CompilerFlag)

where, normally,OptionalRelease is less thanMandatoryRelease, and both are5-tuples of the same form assys.version_info:

(PY_MAJOR_VERSION,# the 2 in 2.1.0a3; an intPY_MINOR_VERSION,# the 1; an intPY_MICRO_VERSION,# the 0; an intPY_RELEASE_LEVEL,# "alpha", "beta", "candidate" or "final"; stringPY_RELEASE_SERIAL# the 3; an int)

OptionalRelease records the first release in which the feature was accepted.

In the case of aMandatoryRelease that has not yet occurred,MandatoryRelease predicts the release in which the feature will become part ofthe language.

ElseMandatoryRelease records when the feature became part of the language; inreleases at or after that, modules no longer need a future statement to use thefeature in question, but may continue to use such imports.

MandatoryRelease may also beNone, meaning that a planned feature gotdropped.

Instances of class_Feature have two corresponding methods,getOptionalRelease() andgetMandatoryRelease().

CompilerFlag is the (bitfield) flag that should be passed in the fourthargument to the built-in functioncompile() to enable the feature indynamically compiled code. This flag is stored in thecompiler_flagattribute on_Feature instances.

No feature description will ever be deleted from__future__. Since itsintroduction in Python 2.1 the following features have found their way into thelanguage using this mechanism: