
``contextlib`` --- Utilities for ``with``-statement contexts
************************************************************

New in version 2.5.

**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.

See also:

   **PEP 0343** - The "with" statement
      The specification, background, and examples for the Python
      ``with`` statement.
