\begin{seealso}
\seepep{308}{Conditional Expressions}{PEP written by
-Guido van Rossum and Raymond D. Hettinger; implemented by Thomas
+Guido van~Rossum and Raymond D. Hettinger; implemented by Thomas
Wouters.}
\end{seealso}
therefore been removed. This seems like a minor bit of language
trivia, but using generators and \code{try...finally} is actually
necessary in order to implement the \keyword{with} statement
-described by PEP 343. We'll look at this new statement in the following
+described by PEP 343. I'll look at this new statement in the following
section.
Another even more esoteric effect of this change: previously, the
\begin{seealso}
\seepep{342}{Coroutines via Enhanced Generators}{PEP written by
-Guido van Rossum and Phillip J. Eby;
+Guido van~Rossum and Phillip J. Eby;
implemented by Phillip J. Eby. Includes examples of
some fancier uses of generators as coroutines.}
uses \code{try...finally} blocks to ensure that clean-up code is
executed.
-First, I'll discuss the statement as it will commonly be used, and
-then a subsection will examine the implementation details and how to
-write objects (called ``context managers'') that can be used with this
-statement.
+In this section, I'll discuss the statement as it will commonly be
+used. In the next section, I'll examine the implementation details
+and show how to write objects called ``context managers'' and
+``contexts'' for use with this statement.
The \keyword{with} statement is a new control-flow structure whose
basic structure is:
...
\end{verbatim}
-There's a \function{nested(\var{mgr1}, \var{mgr2}, ...)} manager that
-combines a number of context managers so you don't need to write
-nested \keyword{with} statements. This example
-both uses a database transaction and also acquires a thread lock:
+You can also use this decorator to write the \method{__context__()} method
+for a class without creating a new class for the context:
+
+\begin{verbatim}
+class DatabaseConnection:
+
+ @contextmanager
+ def __context__ (self):
+ cursor = self.cursor()
+ try:
+ yield cursor
+ except:
+ self.rollback()
+ raise
+ else:
+ self.commit()
+\end{verbatim}
+
+
+There's a \function{nested(\var{mgr1}, \var{mgr2}, ...)} manager that
+combines a number of context managers so you don't need to write
+nested \keyword{with} statements. This example statement does two
+things, starting a database transaction and acquiring a thread lock:
\begin{verbatim}
lock = threading.Lock()
\begin{seealso}
-\seepep{343}{The ``with'' statement}{PEP written by Guido van Rossum
-and Nick Coghlan; implemented by Mike Bland, Guido van Rossum, and
+\seepep{343}{The ``with'' statement}{PEP written by Guido van~Rossum
+and Nick Coghlan; implemented by Mike Bland, Guido van~Rossum, and
Neal Norwitz. The PEP shows the code generated for a \keyword{with}
statement, which can be helpful in learning how context managers
work.}
\begin{seealso}
\seepep{352}{Required Superclass for Exceptions}{PEP written by
-Brett Cannon and Guido van Rossum; implemented by Brett Cannon.}
+Brett Cannon and Guido van~Rossum; implemented by Brett Cannon.}
\end{seealso}
% the cPickle module no longer accepts the deprecated None option in the
% args tuple returned by __reduce__().
-% XXX datetime.datetime() now has a strptime class method which can be used to
-% create datetime object using a string and format.
-
% XXX fileinput: opening hook used to control how files are opened.
% .input() now has a mode parameter
% now has a fileno() function
\member{line_num} is not the same as the number of records read.
(Contributed by Skip Montanaro and Andrew McNamara.)
+\item The \class{datetime} class in the \module{datetime}
+module now has a \method{strptime(\var{string}, \var{format})}
+method for parsing date strings, contributed by Josh Spoerri.
+It uses the same format characters as \function{time.strptime()} and
+\function{time.strftime()}:
+
+\begin{verbatim}
+from datetime import datetime
+
+ts = datetime.strptime('10:13:15 2006-03-07',
+ '%H:%M:%S %Y-%m-%d')
+\end{verbatim}
+
\item In the \module{gc} module, the new \function{get_count()} function
returns a 3-tuple containing the current collection counts for the
three GC generations. This is accounting information for the garbage
The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
-article: Martin von~L\"owis, Mike Rovner, Thomas Wouters.
+article: Phillip J. Eby, Kent Johnson, Martin von~L\"owis, Mike
+Rovner, Thomas Wouters.
\end{document}
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