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  r73465 | nick.coghlan | 2009-06-17 22:12:15 +1000 (Wed, 17 Jun 2009) | 1 line

  Issue 6288: update the contextlib.nested() docs to explain why it has been deprecated and should generally be avoided
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Nick Coghlan 2009-06-17 12:23:43 +00:00
parent e810e5a03d
commit 4958cba231
1 changed files with 29 additions and 11 deletions
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40
Doc/library/contextlib.rst
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@ -56,19 +56,16 @@ Functions provided:
Combine multiple context managers into a single nested context manager. Combine multiple context managers into a single nested context manager.
Code like this:: This function has been deprecated in favour of the multiple manager form
of the :keyword:`with` statement.
The one advantage of this function over the multiple manager form of the
:keyword:`with` statement is that argument unpacking allows it to be
used with a variable number of context managers as follows::
from contextlib import nested from contextlib import nested
with nested(A(), B(), C()) as (X, Y, Z): with nested(*managers):
do_something()
is equivalent to this::
m1, m2, m3 = A(), B(), C()
with m1 as X:
with m2 as Y:
with m3 as Z:
do_something() do_something()
Note that if the :meth:`__exit__` method of one of the nested context managers Note that if the :meth:`__exit__` method of one of the nested context managers
@ -80,8 +77,29 @@ Functions provided:
:meth:`__exit__` methods should avoid raising exceptions, and in particular they :meth:`__exit__` methods should avoid raising exceptions, and in particular they
should not re-raise a passed-in exception. 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
:meth:`__new__` and :meth:`__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 :func:`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 :meth:`__enter__` method of one of the inner context managers
raises an exception that is caught and suppressed by the :meth:`__exit__` method
of one of the outer context managers, this construct will raise
:exc:`RuntimeError` rather than skipping the body of the :keyword:`with`
statement.
Developers that need to support nesting of a variable number of context managers
can either use the :mod:`warnings` module to suppress the DeprecationWarning
raised by this function or else use this function as a model for an application
specific implementation.
.. deprecated:: 3.1 .. deprecated:: 3.1
The with-statement now supports this functionality directly. The with-statement now supports this functionality directly (without the
confusing error prone quirks).
.. function:: closing(thing) .. function:: closing(thing)