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Doc/ref/refa1.tex

-\chapter{Future statements and nested scopes \label{futures}}
-\sectionauthor{Jeremy Hylton}{jeremy@alum.mit.edu}
-
-
-The semantics of Python's static scoping will change in version 2.2 to
-support resolution of unbound local names in enclosing functions'
-namespaces.  The new semantics will be available in Python 2.1 through
-the use of a future statement.  This appendix documents these two
-features for Python 2.1; it will be removed in Python 2.2 and the
-features will be documented in the main sections of this manual.
-
-
-\section{Future statements \label{future-statements}}
-
-A \dfn{future statement}\indexii{future}{statement} is a directive to
-the compiler that a particular module should be compiled using syntax
-or semantics that will be available in a specified future release of
-Python.  The future statement is intended to ease migration to future
-versions of Python that introduce incompatible changes to the
-language.  It allows use of the new features on a per-module basis
-before the release in which the feature becomes standard.
-
-\begin{productionlist}[*]
-  \production{future_statement}
-             {"from" "__future__" "import" feature ["as" name]}
-  \productioncont{("," feature ["as" name])*}
-  \production{feature}{identifier}
-  \production{name}{identifier}
-\end{productionlist}
-
-A future statement must appear near the top of the module.  The only
-lines that can appear before a future statement are:
-
-\begin{itemize}
-
-\item the module docstring (if any),
-\item comments,
-\item blank lines, and
-\item other future statements.
-
-\end{itemize}
-
-The features recognized by Python 2.3 are \samp{generators},
-\samp{division} and \samp{nested_scopes}.  \samp{generators} and
-\samp{nested_scopes} are redundant in 2.3 because they are always
-enabled. 
-
-A future statement is recognized and treated specially at compile
-time: Changes to the semantics of core constructs are often
-implemented by generating different code.  It may even be the case
-that a new feature introduces new incompatible syntax (such as a new
-reserved word), in which case the compiler may need to parse the
-module differently.  Such decisions cannot be pushed off until
-runtime.
-
-For any given release, the compiler knows which feature names have been
-defined, and raises a compile-time error if a future statement contains
-a feature not known to it.
-
-The direct runtime semantics are the same as for any import statement:
-there is a standard module \module{__future__}, described later, and
-it will be imported in the usual way at the time the future statement
-is executed.
-
-The interesting runtime semantics depend on the specific feature
-enabled by the future statement.
-
-Note that there is nothing special about the statement:
-
-\begin{verbatim}
-import __future__ [as name]
-\end{verbatim}
-
-That is not a future statement; it's an ordinary import statement with
-no special semantics or syntax restrictions.
-
-Code compiled by an exec statement or calls to the builtin functions
-\function{compile()} and \function{execfile()} that occur in a module
-\module{M} containing a future statement will, by default, use the new 
-syntax or semantics associated with the future statement.  This can,
-starting with Python 2.2 be controlled by optional arguments to
-\function{compile()} --- see the documentation of that function in the 
-library reference for details.
-
-A future statement typed at an interactive interpreter prompt will
-take effect for the rest of the interpreter session.  If an
-interpreter is started with the \programopt{-i} option, is passed a
-script name to execute, and the script includes a future statement, it
-will be in effect in the interactive session started after the script
-is executed.
-
-\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__}.
-