28.7. contextlib — Utilities for with-statement contexts

Source code: Lib/contextlib.py

---

This module provides utilities for common tasks involving the with statement. For more information see also Context Manager Types and With Statement Context Managers.

Functions provided:

contextlib.contextmanager(func)

This function is a decorator that can be used to define a factory function for with statement context managers, without needing to create a class or separate __enter__() and __exit__() methods.

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>

The function being decorated must return a generator-iterator when called. This iterator must yield exactly one value, which will be bound to the targets in the with statement’s as clause, if any.

At the point where the generator yields, the block nested in the 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 try...except...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 generator context manager will indicate to the with statement that the exception has been handled, and execution will resume with the statement immediately following the with statement.

contextlib.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 with statement.

The one advantage of this function over the multiple manager form of the with statement 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 __new__() and __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 RuntimeError rather than skipping the body of the with statement.

Developers that need to support nesting of a variable number of context managers can either use the warnings module 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).

contextlib.closing(thing)

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 with block is exited.