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).