2 :mod:`contextlib` --- Utilities for :keyword:`with`\ -statement contexts.
3 =========================================================================
6 :synopsis: Utilities for with-statement contexts.
11 This module provides utilities for common tasks involving the :keyword:`with`
12 statement. For more information see also :ref:`typecontextmanager` and
13 :ref:`context-managers`.
18 .. function:: contextmanager(func)
20 This function is a :term:`decorator` that can be used to define a factory
21 function for :keyword:`with` statement context managers, without needing to
22 create a class or separate :meth:`__enter__` and :meth:`__exit__` methods.
24 A simple example (this is not recommended as a real way of generating HTML!)::
26 from contextlib import contextmanager
41 The function being decorated must return a :term:`generator`-iterator when
42 called. This iterator must yield exactly one value, which will be bound to
43 the targets in the :keyword:`with` statement's :keyword:`as` clause, if any.
45 At the point where the generator yields, the block nested in the :keyword:`with`
46 statement is executed. The generator is then resumed after the block is exited.
47 If an unhandled exception occurs in the block, it is reraised inside the
48 generator at the point where the yield occurred. Thus, you can use a
49 :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` statement to trap
50 the error (if any), or ensure that some cleanup takes place. If an exception is
51 trapped merely in order to log it or to perform some action (rather than to
52 suppress it entirely), the generator must reraise that exception. Otherwise the
53 generator context manager will indicate to the :keyword:`with` statement that
54 the exception has been handled, and execution will resume with the statement
55 immediately following the :keyword:`with` statement.
58 .. function:: nested(mgr1[, mgr2[, ...]])
60 Combine multiple context managers into a single nested context manager.
64 from contextlib import nested
66 with nested(A, B, C) as (X, Y, Z):
69 is equivalent to this::
76 Note that if the :meth:`__exit__` method of one of the nested context managers
77 indicates an exception should be suppressed, no exception information will be
78 passed to any remaining outer context managers. Similarly, if the
79 :meth:`__exit__` method of one of the nested managers raises an exception, any
80 previous exception state will be lost; the new exception will be passed to the
81 :meth:`__exit__` methods of any remaining outer context managers. In general,
82 :meth:`__exit__` methods should avoid raising exceptions, and in particular they
83 should not re-raise a passed-in exception.
86 .. function:: closing(thing)
88 Return a context manager that closes *thing* upon completion of the block. This
89 is basically equivalent to::
91 from contextlib import contextmanager
100 And lets you write code like this::
102 from contextlib import closing
105 with closing(urllib.urlopen('http://www.python.org')) as page:
109 without needing to explicitly close ``page``. Even if an error occurs,
110 ``page.close()`` will be called when the :keyword:`with` block is exited.
115 :pep:`0343` - The "with" statement
116 The specification, background, and examples for the Python :keyword:`with`