\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 featurewas accepted. In the case of a \var{MandatoryRelease} that has not yet occurred,\var{MandatoryRelease} predicts the release in which the feature willbecome part of the language. Else \var{MandatoryRelease} records when the feature became part ofthe language; in releases at or after that, modules no longer need afuture statement to use the feature in question, but may continue touse such imports. \var{MandatoryRelease} may also be \code{None}, meaning that a plannedfeature 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 thefourth argument to the builtin function \function{compile()} to enablethe 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__}.