source: trunk/essentials/dev-lang/python/Doc/lib/libcontextlib.tex@ 3368

\section{\module{contextlib} ---
         Utilities for \keyword{with}-statement contexts.}

\declaremodule{standard}{contextlib}
\modulesynopsis{Utilities for \keyword{with}-statement contexts.}

\versionadded{2.5}

This module provides utilities for common tasks involving the
\keyword{with} statement.

Functions provided:

\begin{funcdesc}{contextmanager}{func}
This function is a decorator that can be used to define a factory
function for \keyword{with} statement context managers, without
needing to create a class or separate \method{__enter__()} and
\method{__exit__()} methods.

A simple example (this is not recommended as a real way of
generating HTML!):

\begin{verbatim}
from __future__ import with_statement
from contextlib import contextmanager

@contextmanager
def tag(name):
    print "<%s>" % name
    yield
    print "</%s>" % name

>>> with tag("h1"):
...    print "foo"
...
<h1>
foo
</h1>
\end{verbatim}

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
after the block is exited.  If an unhandled exception occurs in the
block, it is reraised inside the generator at the point where the yield
occurred.  Thus, you can use a
\keyword{try}...\keyword{except}...\keyword{finally} statement to trap
the error (if any), or ensure that some cleanup takes place. If an
exception is trapped merely in order to log it or to perform some
action (rather than to suppress it entirely), the generator must
reraise that exception. Otherwise the generator context manager will
indicate to the \keyword{with} statement that the exception has been
handled, and execution will resume with the statement immediately
following the \keyword{with} statement.
\end{funcdesc}

\begin{funcdesc}{nested}{mgr1\optional{, mgr2\optional{, ...}}}
Combine multiple context managers into a single nested context manager.

Code like this:

\begin{verbatim}
from contextlib import nested

with nested(A, B, C) as (X, Y, Z):
    do_something()
\end{verbatim}

is equivalent to this:

\begin{verbatim}
with A as X:
    with B as Y:
        with C as Z:
            do_something()
\end{verbatim}

Note that if the \method{__exit__()} method of one of the nested
context managers 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 managers 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.
In general, \method{__exit__()} methods should avoid raising
exceptions, and in particular they should not re-raise a
passed-in exception.
\end{funcdesc}

\label{context-closing}
\begin{funcdesc}{closing}{thing}
Return a context manager that closes \var{thing} upon completion of
the block.  This is basically equivalent to:

\begin{verbatim}
from contextlib import contextmanager

@contextmanager
def closing(thing):
    try:
        yield thing
    finally:
        thing.close()
\end{verbatim}

And lets you write code like this:
\begin{verbatim}
from __future__ import with_statement
from contextlib import closing
import codecs

with closing(urllib.urlopen('http://www.python.org')) as page:
    for line in page:
        print line
\end{verbatim}

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.
\end{funcdesc}

\begin{seealso}
  \seepep{0343}{The "with" statement}
         {The specification, background, and examples for the
          Python \keyword{with} statement.}
\end{seealso}