Document the contextlib module.

Doc/lib/lib.tex

@@ -371,6 +371,7 @@ and how to embed it in other applications.
 \input{libbltin}                % really __builtin__
 \input{libmain}                 % really __main__
 \input{libwarnings}
+\input{libcontextlib}
 \input{libatexit}
 \input{libtraceback}
 \input{libfuture}               % really __future__

Doc/lib/libcontextlib.tex

+\section{\module{contextlib} ---
+         Utilities for \keyword{with}-statement contexts.}
+
+\declaremodule{standard}{contextlib}
+\modulesynopsis{Utilities for \keyword{with}-statement contexts.}
+
+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.
+
+Note that you can use \code{@contextmanager} to define a context
+manager's \method{__context__} method.  This is usually more convenient
+than creating another class just to serve as a context.  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 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 one of the nested contexts' \method{__exit__()} method
+raises an exception, any previous exception state will be lost; the new
+exception will be passed to the outer contexts' \method{__exit__()}
+method(s), if any.  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 contextlib import closing
+
+with closing(codecs.open("foo", encoding="utf8")) as f:
+    for line in f:
+        print line.encode("latin1")
+\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}