| author | Deepak Modgil <Deepak.Modgil@Nokia.com> |
| Fri, 03 Apr 2009 17:19:34 +0100 | |
| changeset 0 | ae805ac0140d |
| permissions | -rw-r--r-- |
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ae805ac0140d
DP tools release version Revision: 200912
Deepak Modgil <Deepak.Modgil@Nokia.com>
parents:
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1 |
"""Utilities for with-statement contexts. See PEP 343.""" |
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3 |
import sys |
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__all__ = ["contextmanager", "nested", "closing"] |
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class GeneratorContextManager(object): |
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"""Helper for @contextmanager decorator.""" |
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def __init__(self, gen): |
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self.gen = gen |
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def __enter__(self): |
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try: |
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return self.gen.next() |
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except StopIteration: |
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raise RuntimeError("generator didn't yield")
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def __exit__(self, type, value, traceback): |
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if type is None: |
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try: |
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self.gen.next() |
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except StopIteration: |
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return |
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else: |
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raise RuntimeError("generator didn't stop")
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else: |
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if value is None: |
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# Need to force instantiation so we can reliably |
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# tell if we get the same exception back |
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value = type() |
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try: |
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self.gen.throw(type, value, traceback) |
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raise RuntimeError("generator didn't stop after throw()")
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except StopIteration, exc: |
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# Suppress the exception *unless* it's the same exception that |
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# was passed to throw(). This prevents a StopIteration |
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# raised inside the "with" statement from being suppressed |
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return exc is not value |
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except: |
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# only re-raise if it's *not* the exception that was |
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# passed to throw(), because __exit__() must not raise |
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# an exception unless __exit__() itself failed. But throw() |
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# has to raise the exception to signal propagation, so this |
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# fixes the impedance mismatch between the throw() protocol |
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# and the __exit__() protocol. |
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# |
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if sys.exc_info()[1] is not value: |
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raise |
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def contextmanager(func): |
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"""@contextmanager decorator. |
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54 |
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Typical usage: |
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@contextmanager |
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def some_generator(<arguments>): |
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<setup> |
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try: |
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yield <value> |
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finally: |
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<cleanup> |
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This makes this: |
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66 |
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with some_generator(<arguments>) as <variable>: |
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68 |
<body> |
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equivalent to this: |
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71 |
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<setup> |
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try: |
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<variable> = <value> |
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<body> |
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finally: |
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<cleanup> |
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78 |
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""" |
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80 |
def helper(*args, **kwds): |
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return GeneratorContextManager(func(*args, **kwds)) |
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try: |
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helper.__name__ = func.__name__ |
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helper.__doc__ = func.__doc__ |
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85 |
helper.__dict__ = func.__dict__ |
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except: |
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pass |
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return helper |
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89 |
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90 |
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91 |
@contextmanager |
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92 |
def nested(*managers): |
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93 |
"""Support multiple context managers in a single with-statement. |
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94 |
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95 |
Code like this: |
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96 |
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97 |
with nested(A, B, C) as (X, Y, Z): |
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98 |
<body> |
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99 |
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100 |
is equivalent to this: |
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101 |
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102 |
with A as X: |
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103 |
with B as Y: |
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104 |
with C as Z: |
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105 |
<body> |
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106 |
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107 |
""" |
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108 |
exits = [] |
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109 |
vars = [] |
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110 |
exc = (None, None, None) |
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111 |
try: |
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112 |
try: |
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113 |
for mgr in managers: |
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114 |
exit = mgr.__exit__ |
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115 |
enter = mgr.__enter__ |
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116 |
vars.append(enter()) |
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117 |
exits.append(exit) |
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118 |
yield vars |
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119 |
except: |
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120 |
exc = sys.exc_info() |
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121 |
finally: |
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122 |
while exits: |
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exit = exits.pop() |
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124 |
try: |
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if exit(*exc): |
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exc = (None, None, None) |
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except: |
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exc = sys.exc_info() |
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if exc != (None, None, None): |
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# Don't rely on sys.exc_info() still containing |
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# the right information. Another exception may |
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# have been raised and caught by an exit method |
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raise exc[0], exc[1], exc[2] |
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134 |
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135 |
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class closing(object): |
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"""Context to automatically close something at the end of a block. |
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138 |
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Code like this: |
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140 |
|
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with closing(<module>.open(<arguments>)) as f: |
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<block> |
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143 |
|
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is equivalent to this: |
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145 |
|
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f = <module>.open(<arguments>) |
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try: |
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<block> |
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finally: |
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f.close() |
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151 |
|
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""" |
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def __init__(self, thing): |
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self.thing = thing |
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def __enter__(self): |
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return self.thing |
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def __exit__(self, *exc_info): |
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self.thing.close() |