with/as Context Managers

Python 2.6 and 3.0 introduced a new exception-related statement—the with, and its optional as clause. This statement is designed to work with context manager objects, which support a new method-based protocol. This feature is also available as an option in 2.5, enabled with an import of this form:

from __future__ import with_statement

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In short, the with/as statement is designed to be an alternative to a common try/finally usage idiom; like that statement, it is intended for specifying termination-time or “cleanup” activities that must run regardless of whether an exception occurs in a processing step. Unlike try/finally, though, the with statement supports a richer object-based protocol for specifying both entry and exit actions around a block of code.

Python enhances some built-in tools with context managers, such as files that automatically close themselves and thread locks that automatically lock and unlock, but programmers can code context managers of their own with classes, too.