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gerrit-public.fairphone.software / platform / external / python / cpython3 / 0a27e3db50193de0881d98f9bc856e18f079974e / . / Lib / contextlib.py
blob: 0841ac33ed541611b768c37843f3c629555fdf7b [file] [log] [blame]
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]1"""Utilities for with-statement contexts. See PEP 343."""
2
3import sys
Christian Heimes81ee3ef2008-05-04 22:42:01 +0000[diff] [blame]4from functools import wraps
Raymond Hettinger91e3b9d2009-05-28 22:20:03 +0000[diff] [blame]5from warnings import warn
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]6
7__all__ = ["contextmanager", "nested", "closing"]
8
9class GeneratorContextManager(object):
10 """Helper for @contextmanager decorator."""
11
12 def __init__(self, gen):
13 self.gen = gen
14
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]15 def __enter__(self):
16 try:
Georg Brandla18af4e2007-04-21 15:47:16 +0000[diff] [blame]17 return next(self.gen)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]18 except StopIteration:
19 raise RuntimeError("generator didn't yield")
20
21 def __exit__(self, type, value, traceback):
22 if type is None:
23 try:
Georg Brandla18af4e2007-04-21 15:47:16 +0000[diff] [blame]24 next(self.gen)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]25 except StopIteration:
26 return
27 else:
28 raise RuntimeError("generator didn't stop")
29 else:
Guido van Rossum2cc30da2007-11-02 23:46:40 +0000[diff] [blame]30 if value is None:
31 # Need to force instantiation so we can reliably
32 # tell if we get the same exception back
33 value = type()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]34 try:
35 self.gen.throw(type, value, traceback)
Thomas Wouters49fd7fa2006-04-21 10:40:58 +0000[diff] [blame]36 raise RuntimeError("generator didn't stop after throw()")
Guido van Rossumb940e112007-01-10 16:19:56 +0000[diff] [blame]37 except StopIteration as exc:
Thomas Wouters49fd7fa2006-04-21 10:40:58 +0000[diff] [blame]38 # Suppress the exception *unless* it's the same exception that
39 # was passed to throw(). This prevents a StopIteration
40 # raised inside the "with" statement from being suppressed
41 return exc is not value
42 except:
43 # only re-raise if it's *not* the exception that was
44 # passed to throw(), because __exit__() must not raise
45 # an exception unless __exit__() itself failed. But throw()
46 # has to raise the exception to signal propagation, so this
47 # fixes the impedance mismatch between the throw() protocol
48 # and the __exit__() protocol.
49 #
50 if sys.exc_info()[1] is not value:
51 raise
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]52
53
54def contextmanager(func):
55 """@contextmanager decorator.
56
57 Typical usage:
58
59 @contextmanager
60 def some_generator(<arguments>):
61 <setup>
62 try:
63 yield <value>
64 finally:
65 <cleanup>
66
67 This makes this:
68
69 with some_generator(<arguments>) as <variable>:
70 <body>
71
72 equivalent to this:
73
74 <setup>
75 try:
76 <variable> = <value>
77 <body>
78 finally:
79 <cleanup>
80
81 """
Christian Heimes81ee3ef2008-05-04 22:42:01 +0000[diff] [blame]82 @wraps(func)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]83 def helper(*args, **kwds):
84 return GeneratorContextManager(func(*args, **kwds))
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]85 return helper
86
87
88@contextmanager
Thomas Wouters477c8d52006-05-27 19:21:47 +0000[diff] [blame]89def nested(*managers):
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]90 """Support multiple context managers in a single with-statement.
91
92 Code like this:
93
94 with nested(A, B, C) as (X, Y, Z):
95 <body>
96
97 is equivalent to this:
98
99 with A as X:
100 with B as Y:
101 with C as Z:
102 <body>
103
104 """
Raymond Hettinger91e3b9d2009-05-28 22:20:03 +0000[diff] [blame]105 warn("With-statements now directly support multiple context managers",
106 DeprecationWarning, 2)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]107 exits = []
108 vars = []
Guido van Rossumf6694362006-03-10 02:28:35 +0000[diff] [blame]109 exc = (None, None, None)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]110 try:
Guido van Rossum04110fb2007-08-24 16:32:05 +0000[diff] [blame]111 for mgr in managers:
112 exit = mgr.__exit__
113 enter = mgr.__enter__
114 vars.append(enter())
115 exits.append(exit)
116 yield vars
117 except:
118 exc = sys.exc_info()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]119 finally:
120 while exits:
121 exit = exits.pop()
122 try:
Guido van Rossumf6694362006-03-10 02:28:35 +0000[diff] [blame]123 if exit(*exc):
124 exc = (None, None, None)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]125 except:
126 exc = sys.exc_info()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]127 if exc != (None, None, None):
Thomas Wouters477c8d52006-05-27 19:21:47 +0000[diff] [blame]128 # Don't rely on sys.exc_info() still containing
129 # the right information. Another exception may
130 # have been raised and caught by an exit method
Collin Winterc59dacd2007-09-01 20:27:58 +0000[diff] [blame]131 # exc[1] already has the __traceback__ attribute populated
132 raise exc[1]
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]133
134
Thomas Wouters477c8d52006-05-27 19:21:47 +0000[diff] [blame]135class closing(object):
136 """Context to automatically close something at the end of a block.
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]137
138 Code like this:
139
140 with closing(<module>.open(<arguments>)) as f:
141 <block>
142
143 is equivalent to this:
144
145 f = <module>.open(<arguments>)
146 try:
147 <block>
148 finally:
149 f.close()
150
151 """
Thomas Wouters477c8d52006-05-27 19:21:47 +0000[diff] [blame]152 def __init__(self, thing):
153 self.thing = thing
154 def __enter__(self):
155 return self.thing
156 def __exit__(self, *exc_info):
157 self.thing.close()