1 :mod:`contextlib` --- Utilities for :keyword:`with`\ -statement contexts
2 ========================================================================
5 :synopsis: Utilities for with-statement contexts.
10 This module provides utilities for common tasks involving the :keyword:`with`
11 statement. For more information see also :ref:`typecontextmanager` and
12 :ref:`context-managers`.
17 .. function:: contextmanager(func)
19 This function is a :term:`decorator` that can be used to define a factory
20 function for :keyword:`with` statement context managers, without needing to
21 create a class or separate :meth:`__enter__` and :meth:`__exit__` methods.
23 A simple example (this is not recommended as a real way of generating HTML!)::
25 from contextlib import contextmanager
40 The function being decorated must return a :term:`generator`-iterator when
41 called. This iterator must yield exactly one value, which will be bound to
42 the targets in the :keyword:`with` statement's :keyword:`as` clause, if any.
44 At the point where the generator yields, the block nested in the :keyword:`with`
45 statement is executed. The generator is then resumed after the block is exited.
46 If an unhandled exception occurs in the block, it is reraised inside the
47 generator at the point where the yield occurred. Thus, you can use a
48 :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` statement to trap
49 the error (if any), or ensure that some cleanup takes place. If an exception is
50 trapped merely in order to log it or to perform some action (rather than to
51 suppress it entirely), the generator must reraise that exception. Otherwise the
52 generator context manager will indicate to the :keyword:`with` statement that
53 the exception has been handled, and execution will resume with the statement
54 immediately following the :keyword:`with` statement.
57 .. function:: nested(mgr1[, mgr2[, ...]])
59 Combine multiple context managers into a single nested context manager.
63 from contextlib import nested
65 with nested(A(), B(), C()) as (X, Y, Z):
68 is equivalent to this::
70 m1, m2, m3 = A(), B(), C()
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 The with-statement now supports this functionality directly.
88 .. function:: closing(thing)
90 Return a context manager that closes *thing* upon completion of the block. This
91 is basically equivalent to::
93 from contextlib import contextmanager
102 And lets you write code like this::
104 from contextlib import closing
107 with closing(urllib.urlopen('http://www.python.org')) as page:
111 without needing to explicitly close ``page``. Even if an error occurs,
112 ``page.close()`` will be called when the :keyword:`with` block is exited.
117 :pep:`0343` - The "with" statement
118 The specification, background, and examples for the Python :keyword:`with`