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\chapter{Appendix: Future statements and nested scopes} \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 The semantics of Python's static scoping will change in version 2.2 to
support resolution of unbound local names in enclosing functions' support resolution of unbound local names in enclosing functions'
namespaces. The new semantics will be available in Python 2.1 through namespaces. The new semantics will be available in Python 2.1 through
the use of a future statement. This appendix documents these two 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 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. features will be documented in the main sections of this manual.
\section{Future statements}
\indexii{future}{statement}
A \dfn{future statement} is a directive to the compiler that a \section{Future statements \label{future-statements}}
particular module should be compiled using syntax or semantics that
will be available in a specified future release of Python. The future A \dfn{future statement}\indexii{future}{statement} is a directive to
statement is intended to ease migration to future versions of Python the compiler that a particular module should be compiled using syntax
that introduce incompatible changes to the language. It allows use of or semantics that will be available in a specified future release of
the new features on a per-module basis before the release in which the Python. The future statement is intended to ease migration to future
feature becomes standard. 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{verbatim} \begin{verbatim}
future_statement: "from" "__future__" "import" feature ["as" name] future_statement: "from" "__future__" "import" feature ["as" name]
@ -52,7 +54,7 @@ defined, and raises a compile-time error if a future statement contains
a feature not known to it. a feature not known to it.
The direct runtime semantics are the same as for any import statement: The direct runtime semantics are the same as for any import statement:
there is a standard module \file{__future__.py}, described later, and there is a standard module \module{__future__}, described later, and
it will be imported in the usual way at the time the future statement it will be imported in the usual way at the time the future statement
is executed. is executed.
@ -65,17 +67,17 @@ Note that there is nothing special about the statement:
import __future__ [as name] import __future__ [as name]
\end{verbatim} \end{verbatim}
That is not a future statement; it's an ordinary import statement, with That is not a future statement; it's an ordinary import statement with
no special semantics or syntax restrictions. no special semantics or syntax restrictions.
Code compiled by an exec statement or calls to the builtin functions Code compiled by an exec statement or calls to the builtin functions
\function{compile} and \function{execfile} that occur in a module M \function{compile()} and \function{execfile()} that occur in a module
containing a future statement will use the new syntax or semantics \module{M} containing a future statement will use the new syntax or
associated with the future statement. semantics associated with the future statement.
A future statement typed at an interactive interpreter prompt will A future statement typed at an interactive interpreter prompt will
take effect for the rest of the interpreter session. If an take effect for the rest of the interpreter session. If an
interpreter is started with the \emph{-i} option, is passed a interpreter is started with the \programopt{-i} option, is passed a
script name to execute, and the script includes a future statement, it script name to execute, and the script includes a future statement, it
will be in effect in the interactive session started after the script will be in effect in the interactive session started after the script
is executed. is executed.
@ -83,10 +85,10 @@ is executed.
\section{\module{__future__} --- \section{\module{__future__} ---
Future statement definitions} Future statement definitions}
\declaremodule{standard}{__future__} \declaremodule[future]{standard}{__future__}
\modulesynopsis{Future statement definitions} \modulesynopsis{Future statement definitions}
\file{__future__.py} is a real module, and serves three purposes: \module{__future__} is a real module, and serves three purposes:
\begin{itemize} \begin{itemize}
@ -95,13 +97,13 @@ is executed.
\item To ensure that future_statements run under releases prior to 2.1 \item To ensure that future_statements run under releases prior to 2.1
at least yield runtime exceptions (the import of at least yield runtime exceptions (the import of
\code{__future__} will fail, because there was no module of \module{__future__} will fail, because there was no module of
that name prior to 2.1). that name prior to 2.1).
\item To document when incompatible changes were introduced, and when they \item To document when incompatible changes were introduced, and when they
will be --- or were --- made mandatory. This is a form of executable will be --- or were --- made mandatory. This is a form of executable
documentation, and can be inspected programatically via importing documentation, and can be inspected programatically via importing
\code{__future__} and examining its contents. \module{__future__} and examining its contents.
\end{itemize} \end{itemize}
@ -111,8 +113,8 @@ Each statment in \file{__future__.py} is of the form:
FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ")" FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ")"
\end{verbatim} \end{verbatim}
where, normally, OptionalRelease < MandatoryRelease, and both are where, normally, OptionalRelease is less then MandatoryRelease, and
5-tuples of the same form as \code{sys.version_info}: both are 5-tuples of the same form as \code{sys.version_info}:
\begin{verbatim} \begin{verbatim}
(PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
@ -135,15 +137,17 @@ language; in releases at or after that, modules no longer need a
future statement to use the feature in question, but may continue to future statement to use the feature in question, but may continue to
use such imports. use such imports.
MandatoryRelease may also be None, meaning that a planned feature got MandatoryRelease may also be \code{None}, meaning that a planned
dropped. feature got dropped.
Instances of class \class{_Feature} have two corresponding methods, Instances of class \class{_Feature} have two corresponding methods,
\member{getOptionalRelease()} and \member{getMandatoryRelease()}. \method{getOptionalRelease()} and \method{getMandatoryRelease()}.
No feature line will ever be deleted from \file{__future__.py}. No feature description will ever be deleted from \module{__future__}.
\section{Nested scopes \label{nested-scopes}}
\section{Nested scopes}
\indexii{nested}{scopes} \indexii{nested}{scopes}
Nested scopes are left as an exercise for the reader. Nested scopes are left as an exercise for the reader.