diff options
Diffstat (limited to 'Doc/whatsnew')
| -rw-r--r-- | Doc/whatsnew/whatsnew25.tex | 27 |
1 files changed, 18 insertions, 9 deletions
diff --git a/Doc/whatsnew/whatsnew25.tex b/Doc/whatsnew/whatsnew25.tex index b2f7380..b6bac49 100644 --- a/Doc/whatsnew/whatsnew25.tex +++ b/Doc/whatsnew/whatsnew25.tex @@ -640,15 +640,20 @@ with expression [as variable]: \end{verbatim} The expression is evaluated, and it should result in an object that -supports the context management protocol. This object may return a -value that can optionally be bound to the name \var{variable}. (Note -carefully that \var{variable} is \emph{not} assigned the result of -\var{expression}.) The object can then run set-up code -before \var{with-block} is executed and some clean-up code -is executed after the block is done, even if the block raised an exception. +supports the context management protocol (that is, has \method{__enter__()} +and \method{__exit__()} methods. -To enable the statement in Python 2.5, you need -to add the following directive to your module: +The object's \method{__enter__()} is called before \var{with-block} is +executed and therefore can run set-up code. It also may return a value +that is bound to the name \var{variable}, if given. (Note carefully +that \var{variable} is \emph{not} assigned the result of \var{expression}.) + +After execution of the \var{with-block} is finished, the object's +\method{__exit__()} method is called, even if the block raised an exception, +and can therefore run clean-up code. + +To enable the statement in Python 2.5, you need to add the following +directive to your module: \begin{verbatim} from __future__ import with_statement @@ -668,9 +673,13 @@ with open('/etc/passwd', 'r') as f: \end{verbatim} After this statement has executed, the file object in \var{f} will -have been automatically closed, even if the 'for' loop +have been automatically closed, even if the \keyword{for} loop raised an exception part-way through the block. +\note{In this case, \var{f} is the same object created by + \function{open()}, because \method{file.__enter__()} returns + \var{self}.} + The \module{threading} module's locks and condition variables also support the '\keyword{with}' statement: |