mirror of https://github.com/python/cpython
*** empty log message ***
This commit is contained in:
Jeremy Hylton
parent
bc6e94a0fa
commit
cf9c46719c
|
|
@ -1,160 +0,0 @@
|
|||
\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__}.
|
||||
|
||||
Loading…
Reference in New Issue