contextlib — Utilities for
New in version 2.5.
Source code: Lib/contextlib.py
A simple example (this is not recommended as a real way of generating HTML!):
from contextlib import contextmanager @contextmanager def tag(name): print "<%s>" % name yield print "</%s>" % name >>> with tag("h1"): ... print "foo" ... <h1> foo </h1>
At the point where the generator yields, the block nested in the
withstatement 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
finallystatement 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
withstatement that the exception has been handled, and execution will resume with the statement immediately following the
nested(mgr1[, mgr2[, ...]])
Combine multiple context managers into a single nested context manager.
This function has been deprecated in favour of the multiple manager form of the
The one advantage of this function over the multiple manager form of the
withstatement is that argument unpacking allows it to be used with a variable number of context managers as follows:
from contextlib import nested with nested(*managers): do_something()
Note that if the
__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
__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
__exit__()methods of any remaining outer context managers. In general,
__exit__()methods should avoid raising exceptions, and in particular they should not re-raise a passed-in exception.
This function has two major quirks that have led to it being deprecated. Firstly, as the context managers are all constructed before the function is invoked, the
__init__()methods of the inner context managers are not actually covered by the scope of the outer context managers. That means, for example, that using
nested()to open two files is a programming error as the first file will not be closed promptly if an exception is thrown when opening the second file.
Secondly, if the
__enter__()method of one of the inner context managers raises an exception that is caught and suppressed by the
__exit__()method of one of the outer context managers, this construct will raise
RuntimeErrorrather than skipping the body of the
Developers that need to support nesting of a variable number of context managers can either use the
warningsmodule to suppress the DeprecationWarning raised by this function or else use this function as a model for an application specific implementation.
Deprecated since version 2.7: The with-statement now supports this functionality directly (without the confusing error prone quirks).
Return a context manager that closes thing upon completion of the block. This is basically equivalent to:
from contextlib import contextmanager @contextmanager def closing(thing): try: yield thing finally: thing.close()
And lets you write code like this:
from contextlib import closing import urllib with closing(urllib.urlopen('http://www.python.org')) as page: for line in page: print line
without needing to explicitly close
page. Even if an error occurs,
page.close()will be called when the
withblock is exited.