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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 |
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5 __all__ = ["contextmanager", "nested", "closing"] |
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6 |
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7 class GeneratorContextManager(object): |
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8 """Helper for @contextmanager decorator.""" |
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9 |
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10 def __init__(self, gen): |
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11 self.gen = gen |
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12 |
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13 def __enter__(self): |
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14 try: |
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15 return self.gen.next() |
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16 except StopIteration: |
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17 raise RuntimeError("generator didn't yield") |
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18 |
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19 def __exit__(self, type, value, traceback): |
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20 if type is None: |
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21 try: |
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22 self.gen.next() |
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23 except StopIteration: |
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24 return |
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25 else: |
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26 raise RuntimeError("generator didn't stop") |
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27 else: |
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28 if value is None: |
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29 # Need to force instantiation so we can reliably |
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30 # tell if we get the same exception back |
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31 value = type() |
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32 try: |
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33 self.gen.throw(type, value, traceback) |
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34 raise RuntimeError("generator didn't stop after throw()") |
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35 except StopIteration, exc: |
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36 # Suppress the exception *unless* it's the same exception that |
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37 # was passed to throw(). This prevents a StopIteration |
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38 # raised inside the "with" statement from being suppressed |
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39 return exc is not value |
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40 except: |
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41 # only re-raise if it's *not* the exception that was |
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42 # passed to throw(), because __exit__() must not raise |
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43 # an exception unless __exit__() itself failed. But throw() |
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44 # has to raise the exception to signal propagation, so this |
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45 # fixes the impedance mismatch between the throw() protocol |
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46 # and the __exit__() protocol. |
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47 # |
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48 if sys.exc_info()[1] is not value: |
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49 raise |
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50 |
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51 |
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52 def contextmanager(func): |
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53 """@contextmanager decorator. |
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54 |
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55 Typical usage: |
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56 |
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57 @contextmanager |
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58 def some_generator(<arguments>): |
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59 <setup> |
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60 try: |
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61 yield <value> |
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62 finally: |
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63 <cleanup> |
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64 |
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65 This makes this: |
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66 |
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67 with some_generator(<arguments>) as <variable>: |
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68 <body> |
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69 |
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70 equivalent to this: |
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71 |
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72 <setup> |
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73 try: |
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74 <variable> = <value> |
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75 <body> |
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76 finally: |
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77 <cleanup> |
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78 |
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79 """ |
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80 def helper(*args, **kwds): |
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81 return GeneratorContextManager(func(*args, **kwds)) |
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82 try: |
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83 helper.__name__ = func.__name__ |
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84 helper.__doc__ = func.__doc__ |
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85 helper.__dict__ = func.__dict__ |
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86 except: |
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87 pass |
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88 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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123 exit = exits.pop() |
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124 try: |
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125 if exit(*exc): |
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126 exc = (None, None, None) |
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127 except: |
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128 exc = sys.exc_info() |
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129 if exc != (None, None, None): |
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130 # Don't rely on sys.exc_info() still containing |
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131 # the right information. Another exception may |
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132 # have been raised and caught by an exit method |
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133 raise exc[0], exc[1], exc[2] |
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134 |
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135 |
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136 class closing(object): |
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137 """Context to automatically close something at the end of a block. |
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138 |
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139 Code like this: |
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140 |
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141 with closing(<module>.open(<arguments>)) as f: |
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142 <block> |
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143 |
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144 is equivalent to this: |
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145 |
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146 f = <module>.open(<arguments>) |
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147 try: |
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148 <block> |
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149 finally: |
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150 f.close() |
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151 |
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152 """ |
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153 def __init__(self, thing): |
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154 self.thing = thing |
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155 def __enter__(self): |
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156 return self.thing |
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157 def __exit__(self, *exc_info): |
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158 self.thing.close() |