The with statement 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.
The execution of the with statement proceeds as follows:
The context expression is evaluated to obtain a context manager.
The context manager's __enter__() method is invoked.
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.
The suite is executed.
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.
Note:
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:
