\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 statment in \file{__future__.py} is of the form:

\begin{verbatim}
FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
                        CompilerFlag ")"
\end{verbatim}

where, normally, OptionalRelease is less then 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}

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

In the case of MandatoryReleases that have 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 \code{None}, meaning that a planned
feature got dropped.

Instances of class \class{_Feature} have two corresponding methods,
\method{getOptionalRelease()} and \method{getMandatoryRelease()}.

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{_Future} instances.

No feature description will ever be deleted from \module{__future__}.

