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annotate Lib/contextlib.py @ 49961:e793fc3c8abe

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More 3.0 -> 3.1 transistion in the Windows and OS/2 build files
author Christian Heimes <christian@cheimes.de>
date Thu, 04 Dec 2008 14:27:25 +0000
parents 031dfe455585
children 3eb522dc3337
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 """Utilities for with-statement contexts. See PEP 343."""
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2
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3 import sys
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4 from functools import wraps
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5
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6 __all__ = ["contextmanager", "nested", "closing"]
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7
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8 class GeneratorContextManager(object):
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9 """Helper for @contextmanager decorator."""
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10
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11 def __init__(self, gen):
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12 self.gen = gen
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13
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14 def __enter__(self):
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15 try:
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16 return next(self.gen)
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17 except StopIteration:
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18 raise RuntimeError("generator didn't yield")
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19
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20 def __exit__(self, type, value, traceback):
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21 if type is None:
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22 try:
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23 next(self.gen)
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24 except StopIteration:
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25 return
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26 else:
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27 raise RuntimeError("generator didn't stop")
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28 else:
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29 if value is None:
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30 # Need to force instantiation so we can reliably
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31 # tell if we get the same exception back
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32 value = type()
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33 try:
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34 self.gen.throw(type, value, traceback)
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35 raise RuntimeError("generator didn't stop after throw()")
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36 except StopIteration as exc:
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37 # Suppress the exception *unless* it's the same exception that
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38 # was passed to throw(). This prevents a StopIteration
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39 # raised inside the "with" statement from being suppressed
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40 return exc is not value
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41 except:
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42 # only re-raise if it's *not* the exception that was
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43 # passed to throw(), because __exit__() must not raise
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44 # an exception unless __exit__() itself failed. But throw()
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45 # has to raise the exception to signal propagation, so this
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46 # fixes the impedance mismatch between the throw() protocol
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47 # and the __exit__() protocol.
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48 #
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49 if sys.exc_info()[1] is not value:
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50 raise
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51
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52
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53 def contextmanager(func):
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54 """@contextmanager decorator.
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55
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56 Typical usage:
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57
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58 @contextmanager
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59 def some_generator(<arguments>):
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60 <setup>
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61 try:
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62 yield <value>
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63 finally:
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64 <cleanup>
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65
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66 This makes this:
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67
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68 with some_generator(<arguments>) as <variable>:
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69 <body>
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70
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71 equivalent to this:
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72
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73 <setup>
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74 try:
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75 <variable> = <value>
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76 <body>
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77 finally:
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78 <cleanup>
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79
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80 """
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81 @wraps(func)
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82 def helper(*args, **kwds):
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83 return GeneratorContextManager(func(*args, **kwds))
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84 return helper
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85
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86
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87 @contextmanager
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88 def nested(*managers):
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89 """Support multiple context managers in a single with-statement.
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90
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91 Code like this:
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92
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93 with nested(A, B, C) as (X, Y, Z):
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94 <body>
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95
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96 is equivalent to this:
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97
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98 with A as X:
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99 with B as Y:
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100 with C as Z:
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101 <body>
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102
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103 """
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104 exits = []
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105 vars = []
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106 exc = (None, None, None)
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107 try:
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108 for mgr in managers:
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109 exit = mgr.__exit__
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110 enter = mgr.__enter__
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111 vars.append(enter())
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112 exits.append(exit)
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113 yield vars
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114 except:
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115 exc = sys.exc_info()
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116 finally:
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117 while exits:
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118 exit = exits.pop()
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119 try:
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120 if exit(*exc):
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121 exc = (None, None, None)
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122 except:
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123 exc = sys.exc_info()
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124 if exc != (None, None, None):
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125 # Don't rely on sys.exc_info() still containing
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126 # the right information. Another exception may
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127 # have been raised and caught by an exit method
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128 # exc[1] already has the __traceback__ attribute populated
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129 raise exc[1]
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130
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131
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132 class closing(object):
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133 """Context to automatically close something at the end of a block.
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134
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135 Code like this:
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136
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137 with closing(<module>.open(<arguments>)) as f:
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138 <block>
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139
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140 is equivalent to this:
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141
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142 f = <module>.open(<arguments>)
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143 try:
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144 <block>
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145 finally:
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146 f.close()
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parents:
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147
fa92fe15251a Updates to the with-statement:
Guido van Rossum <guido@python.org>
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148 """
38453
29ae5e0fc348 Much-needed merge (using svnmerge.py this time) of trunk changes into p3yk.
Thomas Wouters <thomas@python.org>
parents: 37879
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149 def __init__(self, thing):
29ae5e0fc348 Much-needed merge (using svnmerge.py this time) of trunk changes into p3yk.
Thomas Wouters <thomas@python.org>
parents: 37879
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150 self.thing = thing
29ae5e0fc348 Much-needed merge (using svnmerge.py this time) of trunk changes into p3yk.
Thomas Wouters <thomas@python.org>
parents: 37879
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151 def __enter__(self):
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Thomas Wouters <thomas@python.org>
parents: 37879
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152 return self.thing
29ae5e0fc348 Much-needed merge (using svnmerge.py this time) of trunk changes into p3yk.
Thomas Wouters <thomas@python.org>
parents: 37879
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153 def __exit__(self, *exc_info):
29ae5e0fc348 Much-needed merge (using svnmerge.py this time) of trunk changes into p3yk.
Thomas Wouters <thomas@python.org>
parents: 37879
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154 self.thing.close()