ref: 2fe3b28f86ed4aa291fb8a620b948abe9c6a1f84
dir: /sys/src/cmd/python/Doc/lib/libfuture.tex/
\section{\module{__future__} --- Future statement definitions} \declaremodule[future]{standard}{__future__} \modulesynopsis{Future statement definitions} \module{__future__} is a real module, and serves three purposes: \begin{itemize} \item To avoid confusing existing tools that analyze import statements and expect to find the modules they're importing. \item To ensure that future_statements run under releases prior to 2.1 at least yield runtime exceptions (the import of \module{__future__} will fail, because there was no module of that name prior to 2.1). \item 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 \module{__future__} and examining its contents. \end{itemize} Each statement in \file{__future__.py} is of the form: \begin{alltt} FeatureName = "_Feature(" \var{OptionalRelease} "," \var{MandatoryRelease} "," \var{CompilerFlag} ")" \end{alltt} where, normally, \var{OptionalRelease} is less than \var{MandatoryRelease}, and both are 5-tuples of the same form as \code{sys.version_info}: \begin{verbatim} (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 ) \end{verbatim} \var{OptionalRelease} records the first release in which the feature was accepted. In the case of a \var{MandatoryRelease} that has not yet occurred, \var{MandatoryRelease} predicts the release in which the feature will become part of the language. Else \var{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. \var{MandatoryRelease} may also be \code{None}, meaning that a planned feature got dropped. Instances of class \class{_Feature} have two corresponding methods, \method{getOptionalRelease()} and \method{getMandatoryRelease()}. \var{CompilerFlag} is the (bitfield) flag that should be passed in the fourth argument to the builtin function \function{compile()} to enable the feature in dynamically compiled code. This flag is stored in the \member{compiler_flag} attribute on \class{_Feature} instances. No feature description will ever be deleted from \module{__future__}.