Doc/lib/libcontextlib.tex

\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 context managers
for use with the \keyword{with} statement, 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

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

>>> with tag("h1"):
...    print "foo"
...
<h1>
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.

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 \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
convenient than creating another class just to serve as a context
manager. For example:

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

class Tag:
    def __init__(self, name):
        self.name = name
        
    @contextmanager
    def __context__(self):
        print "<%s>" % self.name
        yield self
        print "</%s>" % self.name
        
h1 = Tag("h1")

>>> with h1 as me:
...     print "hello from", me
<h1>
hello from <__main__.Tag instance at 0x402ce8ec>
</h1>
\end{verbatim}
\end{funcdesc}

\begin{funcdesc}{nested}{ctx1\optional{, ctx2\optional{, ...}}}
Combine multiple context specifiers 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 context 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 f:
    for line in f:
        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.

\end{funcdesc}

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