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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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7.5 The with statement

The with statement, added in version 2.5, is used to wrap the execution of a block with methods defined by a context manager (see section 3.4.9). This allows common try ... except ... finally usage patterns to be encapsulated for convenient reuse.

with_stmt "with" expression ["as" target] ":"suite

The execution of the with statement proceeds as follows:

  1. The context expression is evaluated to obtain a context manager.
  2. The context manager's __enter__() method is invoked.
  3. If a target was included in the with statement, the return value from __enter__() is assigned to it. Note: The with statement guarantees that if the __enter__() method returns without an error, then __exit__() will always be called. Thus, if an error occurs during the assignment to the target list, it will be treated the same as an error occurring within the suite would be. See step 5 below.
  4. The suite is executed.
  5. The context manager's __exit__() method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to __exit__(). Otherwise, three None arguments are supplied. If the suite was exited due to an exception, and the return value from the __exit__() method was false, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the with statement. If the suite was exited for any reason other than an exception, the return value from __exit__() is ignored, and execution proceeds at the normal location for the kind of exit that was taken.

Notice: In Python 2.5, the with statement is only allowed when the with_statement feature has been enabled. It will always be enabled in Python 2.6. This __future__ import statement can be used to enable the feature:

    from __future__ import with_statement

See also:

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