| 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 self.gen.next()
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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 | self.gen.next()
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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, 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 | raise exc[0], exc[1], exc[2]
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| 129 |
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| 130 |
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| 131 | class closing(object):
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| 132 | """Context to automatically close something at the end of a block.
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| 133 |
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| 134 | Code like this:
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| 135 |
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| 136 | with closing(<module>.open(<arguments>)) as f:
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| 137 | <block>
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| 138 |
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| 139 | is equivalent to this:
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| 140 |
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| 141 | f = <module>.open(<arguments>)
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| 142 | try:
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| 143 | <block>
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| 144 | finally:
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| 145 | f.close()
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| 146 |
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| 147 | """
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| 148 | def __init__(self, thing):
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| 149 | self.thing = thing
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| 150 | def __enter__(self):
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| 151 | return self.thing
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| 152 | def __exit__(self, *exc_info):
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| 153 | self.thing.close()
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