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annotate Lib/contextlib.py @ 87488:124e51c19e4f

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Issue #19734: Also run pip version check in isolated mode
author Nick Coghlan <ncoghlan@gmail.com>
date Sun, 24 Nov 2013 16:49:20 +1000
parents 87d49e2cdd34
children
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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 collections import deque
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5 from functools import wraps
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6
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7 __all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack",
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8 "redirect_stdout", "suppress"]
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9
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10
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11 class ContextDecorator(object):
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12 "A base class or mixin that enables context managers to work as decorators."
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13
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14 def _recreate_cm(self):
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15 """Return a recreated instance of self.
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16
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17 Allows an otherwise one-shot context manager like
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18 _GeneratorContextManager to support use as
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19 a decorator via implicit recreation.
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20
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21 This is a private interface just for _GeneratorContextManager.
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22 See issue #11647 for details.
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23 """
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24 return self
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25
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26 def __call__(self, func):
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27 @wraps(func)
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28 def inner(*args, **kwds):
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29 with self._recreate_cm():
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30 return func(*args, **kwds)
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31 return inner
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32
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33
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34 class _GeneratorContextManager(ContextDecorator):
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35 """Helper for @contextmanager decorator."""
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36
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37 def __init__(self, func, *args, **kwds):
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38 self.gen = func(*args, **kwds)
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39 self.func, self.args, self.kwds = func, args, kwds
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40 # Issue 19330: ensure context manager instances have good docstrings
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41 doc = getattr(func, "__doc__", None)
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42 if doc is None:
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43 doc = type(self).__doc__
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44 self.__doc__ = doc
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45 # Unfortunately, this still doesn't provide good help output when
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46 # inspecting the created context manager instances, since pydoc
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47 # currently bypasses the instance docstring and shows the docstring
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48 # for the class instead.
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49 # See http://bugs.python.org/issue19404 for more details.
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50
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51 def _recreate_cm(self):
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52 # _GCM instances are one-shot context managers, so the
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53 # CM must be recreated each time a decorated function is
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54 # called
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55 return self.__class__(self.func, *self.args, **self.kwds)
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56
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57 def __enter__(self):
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58 try:
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59 return next(self.gen)
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60 except StopIteration:
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61 raise RuntimeError("generator didn't yield") from None
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62
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63 def __exit__(self, type, value, traceback):
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64 if type is None:
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65 try:
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66 next(self.gen)
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67 except StopIteration:
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68 return
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69 else:
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70 raise RuntimeError("generator didn't stop")
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71 else:
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72 if value is None:
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73 # Need to force instantiation so we can reliably
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74 # tell if we get the same exception back
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75 value = type()
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76 try:
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77 self.gen.throw(type, value, traceback)
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78 raise RuntimeError("generator didn't stop after throw()")
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79 except StopIteration as exc:
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80 # Suppress the exception *unless* it's the same exception that
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81 # was passed to throw(). This prevents a StopIteration
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82 # raised inside the "with" statement from being suppressed
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83 return exc is not value
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84 except:
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85 # only re-raise if it's *not* the exception that was
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86 # passed to throw(), because __exit__() must not raise
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87 # an exception unless __exit__() itself failed. But throw()
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88 # has to raise the exception to signal propagation, so this
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89 # fixes the impedance mismatch between the throw() protocol
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90 # and the __exit__() protocol.
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91 #
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92 if sys.exc_info()[1] is not value:
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93 raise
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94
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95
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96 def contextmanager(func):
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97 """@contextmanager decorator.
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98
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99 Typical usage:
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100
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101 @contextmanager
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102 def some_generator(<arguments>):
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103 <setup>
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104 try:
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105 yield <value>
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106 finally:
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107 <cleanup>
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108
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109 This makes this:
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110
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111 with some_generator(<arguments>) as <variable>:
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112 <body>
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113
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114 equivalent to this:
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115
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116 <setup>
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117 try:
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118 <variable> = <value>
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119 <body>
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120 finally:
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121 <cleanup>
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122
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123 """
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124 @wraps(func)
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125 def helper(*args, **kwds):
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126 return _GeneratorContextManager(func, *args, **kwds)
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127 return helper
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128
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129
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130 class closing(object):
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131 """Context to automatically close something at the end of a block.
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132
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133 Code like this:
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134
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135 with closing(<module>.open(<arguments>)) as f:
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136 <block>
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137
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138 is equivalent to this:
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139
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140 f = <module>.open(<arguments>)
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141 try:
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142 <block>
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143 finally:
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144 f.close()
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145
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146 """
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147 def __init__(self, thing):
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148 self.thing = thing
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149 def __enter__(self):
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150 return self.thing
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151 def __exit__(self, *exc_info):
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152 self.thing.close()
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153
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154 class redirect_stdout:
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155 """Context manager for temporarily redirecting stdout to another file
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156
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157 # How to send help() to stderr
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158 with redirect_stdout(sys.stderr):
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159 help(dir)
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160
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161 # How to write help() to a file
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162 with open('help.txt', 'w') as f:
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163 with redirect_stdout(f):
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164 help(pow)
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165 """
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166
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167 def __init__(self, new_target):
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168 self._new_target = new_target
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169 # We use a list of old targets to make this CM re-entrant
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170 self._old_targets = []
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171
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172 def __enter__(self):
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173 self._old_targets.append(sys.stdout)
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174 sys.stdout = self._new_target
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175 return self._new_target
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176
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177 def __exit__(self, exctype, excinst, exctb):
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178 sys.stdout = self._old_targets.pop()
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179
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180
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181 class suppress:
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182 """Context manager to suppress specified exceptions
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183
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184 After the exception is suppressed, execution proceeds with the next
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185 statement following the with statement.
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186
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187 with suppress(FileNotFoundError):
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188 os.remove(somefile)
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189 # Execution still resumes here if the file was already removed
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190 """
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191
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192 def __init__(self, *exceptions):
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193 self._exceptions = exceptions
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194
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195 def __enter__(self):
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196 pass
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197
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198 def __exit__(self, exctype, excinst, exctb):
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199 # Unlike isinstance and issubclass, CPython exception handling
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200 # currently only looks at the concrete type hierarchy (ignoring
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201 # the instance and subclass checking hooks). While Guido considers
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202 # that a bug rather than a feature, it's a fairly hard one to fix
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203 # due to various internal implementation details. suppress provides
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204 # the simpler issubclass based semantics, rather than trying to
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205 # exactly reproduce the limitations of the CPython interpreter.
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206 #
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207 # See http://bugs.python.org/issue12029 for more details
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208 return exctype is not None and issubclass(exctype, self._exceptions)
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209
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210
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211 # Inspired by discussions on http://bugs.python.org/issue13585
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212 class ExitStack(object):
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213 """Context manager for dynamic management of a stack of exit callbacks
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214
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215 For example:
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216
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217 with ExitStack() as stack:
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218 files = [stack.enter_context(open(fname)) for fname in filenames]
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219 # All opened files will automatically be closed at the end of
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220 # the with statement, even if attempts to open files later
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221 # in the list raise an exception
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222
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223 """
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224 def __init__(self):
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225 self._exit_callbacks = deque()
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226
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227 def pop_all(self):
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228 """Preserve the context stack by transferring it to a new instance"""
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229 new_stack = type(self)()
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230 new_stack._exit_callbacks = self._exit_callbacks
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231 self._exit_callbacks = deque()
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232 return new_stack
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233
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234 def _push_cm_exit(self, cm, cm_exit):
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235 """Helper to correctly register callbacks to __exit__ methods"""
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236 def _exit_wrapper(*exc_details):
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237 return cm_exit(cm, *exc_details)
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238 _exit_wrapper.__self__ = cm
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239 self.push(_exit_wrapper)
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240
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241 def push(self, exit):
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242 """Registers a callback with the standard __exit__ method signature
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243
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244 Can suppress exceptions the same way __exit__ methods can.
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245
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246 Also accepts any object with an __exit__ method (registering a call
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247 to the method instead of the object itself)
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248 """
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249 # We use an unbound method rather than a bound method to follow
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250 # the standard lookup behaviour for special methods
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251 _cb_type = type(exit)
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252 try:
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253 exit_method = _cb_type.__exit__
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254 except AttributeError:
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255 # Not a context manager, so assume its a callable
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256 self._exit_callbacks.append(exit)
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257 else:
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258 self._push_cm_exit(exit, exit_method)
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259 return exit # Allow use as a decorator
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260
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261 def callback(self, callback, *args, **kwds):
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262 """Registers an arbitrary callback and arguments.
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263
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264 Cannot suppress exceptions.
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265 """
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266 def _exit_wrapper(exc_type, exc, tb):
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267 callback(*args, **kwds)
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268 # We changed the signature, so using @wraps is not appropriate, but
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269 # setting __wrapped__ may still help with introspection
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270 _exit_wrapper.__wrapped__ = callback
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271 self.push(_exit_wrapper)
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272 return callback # Allow use as a decorator
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273
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274 def enter_context(self, cm):
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275 """Enters the supplied context manager
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276
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277 If successful, also pushes its __exit__ method as a callback and
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278 returns the result of the __enter__ method.
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279 """
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280 # We look up the special methods on the type to match the with statement
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281 _cm_type = type(cm)
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282 _exit = _cm_type.__exit__
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283 result = _cm_type.__enter__(cm)
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284 self._push_cm_exit(cm, _exit)
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285 return result
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286
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287 def close(self):
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288 """Immediately unwind the context stack"""
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289 self.__exit__(None, None, None)
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290
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291 def __enter__(self):
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292 return self
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293
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294 def __exit__(self, *exc_details):
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295 received_exc = exc_details[0] is not None
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296
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297 # We manipulate the exception state so it behaves as though
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298 # we were actually nesting multiple with statements
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299 frame_exc = sys.exc_info()[1]
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300 def _fix_exception_context(new_exc, old_exc):
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301 while 1:
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302 exc_context = new_exc.__context__
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303 if exc_context in (None, frame_exc):
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304 break
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305 new_exc = exc_context
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306 new_exc.__context__ = old_exc
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307
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308 # Callbacks are invoked in LIFO order to match the behaviour of
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309 # nested context managers
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310 suppressed_exc = False
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311 pending_raise = False
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312 while self._exit_callbacks:
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313 cb = self._exit_callbacks.pop()
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314 try:
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315 if cb(*exc_details):
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316 suppressed_exc = True
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317 pending_raise = False
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318 exc_details = (None, None, None)
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319 except:
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320 new_exc_details = sys.exc_info()
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321 # simulate the stack of exceptions by setting the context
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322 _fix_exception_context(new_exc_details[1], exc_details[1])
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323 pending_raise = True
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324 exc_details = new_exc_details
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325 if pending_raise:
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326 try:
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327 # bare "raise exc_details[1]" replaces our carefully
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328 # set-up context
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329 fixed_ctx = exc_details[1].__context__
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330 raise exc_details[1]
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331 except BaseException:
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332 exc_details[1].__context__ = fixed_ctx
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333 raise
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334 return received_exc and suppressed_exc