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gerrit-public.fairphone.software / platform / external / python / cpython2 / b27745fb6d522b18cef532e4b51c9368d568e90e / . / Lib / contextlib.py
blob: dbd1c5744d7a3484bcbf833be465cb42873aba54 [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
4
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]5__all__ = ["contextmanager", "nested", "closing"]
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]6
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]7class GeneratorContextManager(object):
8 """Helper for @contextmanager decorator."""
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]9
10 def __init__(self, gen):
11 self.gen = gen
12
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]13 def __enter__(self):
14 try:
15 return self.gen.next()
16 except StopIteration:
17 raise RuntimeError("generator didn't yield")
18
19 def __exit__(self, type, value, traceback):
20 if type is None:
21 try:
22 self.gen.next()
23 except StopIteration:
24 return
25 else:
26 raise RuntimeError("generator didn't stop")
27 else:
Nick Coghlan3814a912007-11-02 10:09:12 +0000[diff] [blame]28 if value is None:
29 # Need to force instantiation so we can reliably
30 # tell if we get the same exception back
31 value = type()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]32 try:
33 self.gen.throw(type, value, traceback)
Phillip J. Eby6edd2582006-03-25 00:28:24 +0000[diff] [blame]34 raise RuntimeError("generator didn't stop after throw()")
Phillip J. Eby93149d92006-04-10 17:56:29 +0000[diff] [blame]35 except StopIteration, exc:
Phillip J. Eby93880202006-04-03 21:20:07 +0000[diff] [blame]36 # Suppress the exception *unless* it's the same exception that
37 # was passed to throw(). This prevents a StopIteration
38 # raised inside the "with" statement from being suppressed
Phillip J. Eby93149d92006-04-10 17:56:29 +0000[diff] [blame]39 return exc is not value
Phillip J. Eby6edd2582006-03-25 00:28:24 +0000[diff] [blame]40 except:
Phillip J. Ebyccc7bb42006-03-25 04:32:12 +0000[diff] [blame]41 # only re-raise if it's *not* the exception that was
42 # passed to throw(), because __exit__() must not raise
43 # an exception unless __exit__() itself failed. But throw()
44 # has to raise the exception to signal propagation, so this
45 # fixes the impedance mismatch between the throw() protocol
46 # and the __exit__() protocol.
47 #
Phillip J. Eby6edd2582006-03-25 00:28:24 +0000[diff] [blame]48 if sys.exc_info()[1] is not value:
49 raise
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]50
51
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]52def contextmanager(func):
53 """@contextmanager decorator.
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]54
55 Typical usage:
56
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]57 @contextmanager
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]58 def some_generator(<arguments>):
59 <setup>
60 try:
61 yield <value>
62 finally:
63 <cleanup>
64
65 This makes this:
66
67 with some_generator(<arguments>) as <variable>:
68 <body>
69
70 equivalent to this:
71
72 <setup>
73 try:
74 <variable> = <value>
75 <body>
76 finally:
77 <cleanup>
78
79 """
80 def helper(*args, **kwds):
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]81 return GeneratorContextManager(func(*args, **kwds))
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]82 try:
83 helper.__name__ = func.__name__
84 helper.__doc__ = func.__doc__
Phillip J. Eby35fd1422006-03-28 00:07:24 +0000[diff] [blame]85 helper.__dict__ = func.__dict__
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]86 except:
87 pass
88 return helper
89
90
Nick Coghlanafd5e632006-05-03 13:02:47 +0000[diff] [blame]91@contextmanager
Guido van Rossumda5b7012006-05-02 19:47:52 +0000[diff] [blame]92def nested(*managers):
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]93 """Support multiple context managers in a single with-statement.
94
95 Code like this:
96
97 with nested(A, B, C) as (X, Y, Z):
98 <body>
99
100 is equivalent to this:
101
102 with A as X:
103 with B as Y:
104 with C as Z:
105 <body>
106
107 """
108 exits = []
109 vars = []
Guido van Rossumf6694362006-03-10 02:28:35 +0000[diff] [blame]110 exc = (None, None, None)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]111 try:
Georg Brandlac4018a2007-08-23 18:11:33 +0000[diff] [blame]112 for mgr in managers:
113 exit = mgr.__exit__
114 enter = mgr.__enter__
115 vars.append(enter())
116 exits.append(exit)
117 yield vars
118 except:
119 exc = sys.exc_info()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]120 finally:
121 while exits:
122 exit = exits.pop()
123 try:
Guido van Rossumf6694362006-03-10 02:28:35 +0000[diff] [blame]124 if exit(*exc):
125 exc = (None, None, None)
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]126 except:
127 exc = sys.exc_info()
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]128 if exc != (None, None, None):
Nick Coghlanda2268f2006-04-24 04:37:15 +0000[diff] [blame]129 # Don't rely on sys.exc_info() still containing
130 # the right information. Another exception may
131 # have been raised and caught by an exit method
132 raise exc[0], exc[1], exc[2]
Guido van Rossum1a5e21e2006-02-28 21:57:43 +0000[diff] [blame]133
134
Nick Coghlana7e820a2006-04-25 10:56:51 +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 """
Nick Coghlana7e820a2006-04-25 10:56:51 +0000[diff] [blame]152 def __init__(self, thing):
153 self.thing = thing
Nick Coghlana7e820a2006-04-25 10:56:51 +0000[diff] [blame]154 def __enter__(self):
155 return self.thing
156 def __exit__(self, *exc_info):
157 self.thing.close()