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Diffstat (limited to 'Doc/lib/libcontextlib.tex')
-rw-r--r-- | Doc/lib/libcontextlib.tex | 73 |
1 files changed, 47 insertions, 26 deletions
diff --git a/Doc/lib/libcontextlib.tex b/Doc/lib/libcontextlib.tex index 2a9eb0e..f212174 100644 --- a/Doc/lib/libcontextlib.tex +++ b/Doc/lib/libcontextlib.tex @@ -11,18 +11,19 @@ This module provides utilities for common tasks involving the Functions provided: -\begin{funcdesc}{contextmanager}{func} -This function is a decorator that can be used to define context managers -for use with the \keyword{with} statement, without needing to create a -class or separate \method{__enter__()} and \method{__exit__()} methods. +\begin{funcdesc}{context}func} +This function is a decorator that can be used to define a factory +function for \keyword{with} statement context objects, without +needing to create a class or separate \method{__enter__()} and +\method{__exit__()} methods. A simple example: \begin{verbatim} from __future__ import with_statement -from contextlib import contextmanager +from contextlib import contextfactory -@contextmanager +@contextfactory def tag(name): print "<%s>" % name yield @@ -36,9 +37,10 @@ foo </h1> \end{verbatim} -When called, the decorated function must return a generator-iterator. -This iterator must yield exactly one value, which will be bound to the -targets in the \keyword{with} statement's \keyword{as} clause, if any. +The function being decorated must return a generator-iterator when +called. This iterator must yield exactly one value, which will be +bound to the targets in the \keyword{with} statement's \keyword{as} +clause, if any. At the point where the generator yields, the block nested in the \keyword{with} statement is executed. The generator is then resumed @@ -53,20 +55,20 @@ reraise that exception. Otherwise the \keyword{with} statement will treat the exception as having been handled, and resume execution with the statement immediately following the \keyword{with} statement. -Note that you can use \code{@contextmanager} to define a context -specifier's \method{__context__} method. This is usually more +Note that you can use \code{@contextfactory} to define a context +manager's \method{__context__} method. This is usually more convenient than creating another class just to serve as a context -manager. For example: +object. For example: \begin{verbatim} from __future__ import with_statement -from contextlib import contextmanager +from contextlib import contextfactory class Tag: def __init__(self, name): self.name = name - @contextmanager + @contextfactory def __context__(self): print "<%s>" % self.name yield self @@ -83,7 +85,7 @@ hello from <__main__.Tag instance at 0x402ce8ec> \end{funcdesc} \begin{funcdesc}{nested}{ctx1\optional{, ctx2\optional{, ...}}} -Combine multiple context specifiers into a single nested context manager. +Combine multiple context managers into a single nested context manager. Code like this: @@ -104,12 +106,12 @@ with A as X: \end{verbatim} Note that if the \method{__exit__()} method of one of the nested -context managers indicates an exception should be suppressed, no +context objects indicates an exception should be suppressed, no exception information will be passed to any remaining outer context -managers. Similarly, if the \method{__exit__()} method of one of the -nested context managers raises an exception, any previous exception +objects. Similarly, if the \method{__exit__()} method of one of the +nested context objects raises an exception, any previous exception state will be lost; the new exception will be passed to the -\method{__exit__()} methods of any remaining outer context managers. +\method{__exit__()} methods of any remaining outer context objects. In general, \method{__exit__()} methods should avoid raising exceptions, and in particular they should not re-raise a passed-in exception. @@ -117,13 +119,13 @@ passed-in exception. \label{context-closing} \begin{funcdesc}{closing}{thing} -Return a context manager that closes \var{thing} upon completion of the +Return a context that closes \var{thing} upon completion of the block. This is basically equivalent to: \begin{verbatim} -from contextlib import contextmanager +from contextlib import contextfactory -@contextmanager +@contextfactory def closing(thing): try: yield thing @@ -137,14 +139,33 @@ from __future__ import with_statement from contextlib import closing import codecs -with closing(urllib.urlopen('http://www.python.org')) as f: - for line in f: +with closing(urllib.urlopen('http://www.python.org')) as page: + for line in page: print line \end{verbatim} -without needing to explicitly close \code{f}. Even if an error occurs, -\code{f.close()} will be called when the \keyword{with} block is exited. +without needing to explicitly close \code{page}. Even if an error +occurs, \code{page.close()} will be called when the \keyword{with} +block is exited. +Context managers with a close method can use this context factory +directly without needing to implement their own +\method{__context__()} method. +\begin{verbatim} +from __future__ import with_statement +from contextlib import closing + +class MyClass: + def close(self): + print "Closing", self + __context__ = closing + +>>> with MyClass() as x: +... print "Hello from", x +... +Hello from <__main__.MyClass instance at 0xb7df02ec> +Closing <__main__.MyClass instance at 0xb7df02ec> +\end{verbatim} \end{funcdesc} \begin{seealso} |