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annotate Lib/contextlib.py @ 55018:e3fa2ddbb3a3

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Removed contextlib.nested()
author Raymond Hettinger <python@rcn.com>
date Wed, 01 Jul 2009 01:32:12 +0000
parents 45506be44514
children e80859461de8
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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 from warnings import warn
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6
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7 __all__ = ["contextmanager", "nested", "closing"]
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8
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9 class GeneratorContextManager(object):
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10 """Helper for @contextmanager decorator."""
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11
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12 def __init__(self, gen):
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13 self.gen = gen
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14
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15 def __enter__(self):
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16 try:
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17 return next(self.gen)
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18 except StopIteration:
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19 raise RuntimeError("generator didn't yield")
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20
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21 def __exit__(self, type, value, traceback):
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22 if type is None:
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23 try:
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24 next(self.gen)
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25 except StopIteration:
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26 return
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27 else:
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28 raise RuntimeError("generator didn't stop")
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29 else:
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30 if value is None:
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31 # Need to force instantiation so we can reliably
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32 # tell if we get the same exception back
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33 value = type()
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34 try:
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35 self.gen.throw(type, value, traceback)
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36 raise RuntimeError("generator didn't stop after throw()")
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37 except StopIteration as exc:
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38 # Suppress the exception *unless* it's the same exception that
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39 # was passed to throw(). This prevents a StopIteration
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40 # raised inside the "with" statement from being suppressed
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41 return exc is not value
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42 except:
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43 # only re-raise if it's *not* the exception that was
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44 # passed to throw(), because __exit__() must not raise
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45 # an exception unless __exit__() itself failed. But throw()
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46 # has to raise the exception to signal propagation, so this
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47 # fixes the impedance mismatch between the throw() protocol
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48 # and the __exit__() protocol.
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49 #
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50 if sys.exc_info()[1] is not value:
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51 raise
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52
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53
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54 def contextmanager(func):
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55 """@contextmanager decorator.
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56
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57 Typical usage:
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58
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59 @contextmanager
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60 def some_generator(<arguments>):
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61 <setup>
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62 try:
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63 yield <value>
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64 finally:
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65 <cleanup>
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66
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67 This makes this:
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68
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69 with some_generator(<arguments>) as <variable>:
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70 <body>
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71
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72 equivalent to this:
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73
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74 <setup>
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75 try:
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76 <variable> = <value>
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77 <body>
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78 finally:
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79 <cleanup>
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80
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81 """
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82 @wraps(func)
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83 def helper(*args, **kwds):
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84 return GeneratorContextManager(func(*args, **kwds))
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85 return helper
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86
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87
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88 class closing(object):
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89 """Context to automatically close something at the end of a block.
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90
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91 Code like this:
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92
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93 with closing(<module>.open(<arguments>)) as f:
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94 <block>
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95
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96 is equivalent to this:
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97
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98 f = <module>.open(<arguments>)
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99 try:
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100 <block>
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101 finally:
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102 f.close()
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103
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104 """
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105 def __init__(self, thing):
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106 self.thing = thing
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107 def __enter__(self):
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108 return self.thing
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109 def __exit__(self, *exc_info):
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110 self.thing.close()