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author | Jeremy Hylton <jeremy@alum.mit.edu> | 2003-05-28 11:51:11 (GMT) |
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committer | Jeremy Hylton <jeremy@alum.mit.edu> | 2003-05-28 11:51:11 (GMT) |
commit | cf9c46719c1e0d6eeeb4e5329fc10a26d7c8732e (patch) | |
tree | d626838f8da6a852180fd7f924b25b2161b1d4a4 /Doc | |
parent | bc6e94a0fadd5913d8e14453f69cc3942f13ccca (diff) | |
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-rw-r--r-- | Doc/ref/refa1.tex | 160 |
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diff --git a/Doc/ref/refa1.tex b/Doc/ref/refa1.tex deleted file mode 100644 index fe1bc75..0000000 --- a/Doc/ref/refa1.tex +++ /dev/null @@ -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__}. - |