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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
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3.4.9 With Statement Context Managers

Added in version 2.5, a context manager is an object that defines the runtime context to be established when executing a with statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the with statement (described in section 7.5), but can also be used by directly invoking their methods.

Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.

For more information on context managers, see "Context Types" in the Python Library Reference Manual.

__enter__(self)
Enter the runtime context related to this object. The with statement will bind this method's return value to the target(s) specified in the as clause of the statement, if any.
__exit__(self, exc_type, exc_value, traceback)
Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be None. If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method. Note that __exit__ methods should not reraise the passed-in exception; this is the caller's responsibility.

See also:

PEP0343 The "with" statement
The specification, background, and examples for the Python with statement.
ISBN 0954161785Python Language Reference ManualSee the print edition