70 lines
2.5 KiB
TeX
70 lines
2.5 KiB
TeX
\section{\module{__future__} ---
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Future statement definitions}
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\declaremodule[future]{standard}{__future__}
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\modulesynopsis{Future statement definitions}
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\module{__future__} is a real module, and serves three purposes:
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\begin{itemize}
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\item To avoid confusing existing tools that analyze import statements
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and expect to find the modules they're importing.
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\item To ensure that future_statements run under releases prior to 2.1
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at least yield runtime exceptions (the import of
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\module{__future__} will fail, because there was no module of
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that name prior to 2.1).
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\item To document when incompatible changes were introduced, and when they
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will be --- or were --- made mandatory. This is a form of executable
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documentation, and can be inspected programatically via importing
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\module{__future__} and examining its contents.
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\end{itemize}
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Each statement in \file{__future__.py} is of the form:
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\begin{alltt}
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FeatureName = "_Feature(" \var{OptionalRelease} "," \var{MandatoryRelease} ","
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\var{CompilerFlag} ")"
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\end{alltt}
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where, normally, \var{OptionalRelease} is less than
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\var{MandatoryRelease}, and both are 5-tuples of the same form as
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\code{sys.version_info}:
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\begin{verbatim}
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(PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
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PY_MINOR_VERSION, # the 1; an int
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PY_MICRO_VERSION, # the 0; an int
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PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
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PY_RELEASE_SERIAL # the 3; an int
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)
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\end{verbatim}
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\var{OptionalRelease} records the first release in which the feature
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was accepted.
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In the case of a \var{MandatoryRelease} that has not yet occurred,
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\var{MandatoryRelease} predicts the release in which the feature will
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become part of the language.
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Else \var{MandatoryRelease} records when the feature became part of
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the language; in releases at or after that, modules no longer need a
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future statement to use the feature in question, but may continue to
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use such imports.
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\var{MandatoryRelease} may also be \code{None}, meaning that a planned
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feature got dropped.
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Instances of class \class{_Feature} have two corresponding methods,
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\method{getOptionalRelease()} and \method{getMandatoryRelease()}.
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\var{CompilerFlag} is the (bitfield) flag that should be passed in the
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fourth argument to the builtin function \function{compile()} to enable
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the feature in dynamically compiled code. This flag is stored in the
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\member{compiler_flag} attribute on \class{_Feature} instances.
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No feature description will ever be deleted from \module{__future__}.
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