| Tor Norbye | 3a2425a | 2013-11-04 10:16:08 -0800 | [diff] [blame^] | 1 | """Utilities for with-statement contexts. See PEP 343.""" |
| 2 | |
| 3 | import sys |
| 4 | |
| 5 | __all__ = ["contextmanager", "nested", "closing"] |
| 6 | |
| 7 | class GeneratorContextManager(object): |
| 8 | """Helper for @contextmanager decorator.""" |
| 9 | |
| 10 | def __init__(self, gen): |
| 11 | self.gen = gen |
| 12 | |
| 13 | def __enter__(self): |
| 14 | try: |
| 15 | return self.gen.next() |
| 16 | except StopIteration: |
| 17 | raise RuntimeError("generator didn't yield") |
| 18 | |
| 19 | def __exit__(self, type, value, traceback): |
| 20 | if type is None: |
| 21 | try: |
| 22 | self.gen.next() |
| 23 | except StopIteration: |
| 24 | return |
| 25 | else: |
| 26 | raise RuntimeError("generator didn't stop") |
| 27 | else: |
| 28 | if value is None: |
| 29 | # Need to force instantiation so we can reliably |
| 30 | # tell if we get the same exception back |
| 31 | value = type() |
| 32 | try: |
| 33 | self.gen.throw(type, value, traceback) |
| 34 | raise RuntimeError("generator didn't stop after throw()") |
| 35 | except StopIteration, exc: |
| 36 | # Suppress the exception *unless* it's the same exception that |
| 37 | # was passed to throw(). This prevents a StopIteration |
| 38 | # raised inside the "with" statement from being suppressed |
| 39 | return exc is not value |
| 40 | except: |
| 41 | # only re-raise if it's *not* the exception that was |
| 42 | # passed to throw(), because __exit__() must not raise |
| 43 | # an exception unless __exit__() itself failed. But throw() |
| 44 | # has to raise the exception to signal propagation, so this |
| 45 | # fixes the impedance mismatch between the throw() protocol |
| 46 | # and the __exit__() protocol. |
| 47 | # |
| 48 | if sys.exc_info()[1] is not value: |
| 49 | raise |
| 50 | |
| 51 | |
| 52 | def contextmanager(func): |
| 53 | """@contextmanager decorator. |
| 54 | |
| 55 | Typical usage: |
| 56 | |
| 57 | @contextmanager |
| 58 | def some_generator(<arguments>): |
| 59 | <setup> |
| 60 | try: |
| 61 | yield <value> |
| 62 | finally: |
| 63 | <cleanup> |
| 64 | |
| 65 | This makes this: |
| 66 | |
| 67 | with some_generator(<arguments>) as <variable>: |
| 68 | <body> |
| 69 | |
| 70 | equivalent to this: |
| 71 | |
| 72 | <setup> |
| 73 | try: |
| 74 | <variable> = <value> |
| 75 | <body> |
| 76 | finally: |
| 77 | <cleanup> |
| 78 | |
| 79 | """ |
| 80 | def helper(*args, **kwds): |
| 81 | return GeneratorContextManager(func(*args, **kwds)) |
| 82 | try: |
| 83 | helper.__name__ = func.__name__ |
| 84 | helper.__doc__ = func.__doc__ |
| 85 | helper.__dict__ = func.__dict__ |
| 86 | except: |
| 87 | pass |
| 88 | return helper |
| 89 | |
| 90 | |
| 91 | @contextmanager |
| 92 | def nested(*managers): |
| 93 | """Support multiple context managers in a single with-statement. |
| 94 | |
| 95 | Code like this: |
| 96 | |
| 97 | with nested(A, B, C) as (X, Y, Z): |
| 98 | <body> |
| 99 | |
| 100 | is equivalent to this: |
| 101 | |
| 102 | with A as X: |
| 103 | with B as Y: |
| 104 | with C as Z: |
| 105 | <body> |
| 106 | |
| 107 | """ |
| 108 | exits = [] |
| 109 | vars = [] |
| 110 | exc = (None, None, None) |
| 111 | try: |
| 112 | try: |
| 113 | for mgr in managers: |
| 114 | exit = mgr.__exit__ |
| 115 | enter = mgr.__enter__ |
| 116 | vars.append(enter()) |
| 117 | exits.append(exit) |
| 118 | yield vars |
| 119 | except: |
| 120 | exc = sys.exc_info() |
| 121 | finally: |
| 122 | while exits: |
| 123 | exit = exits.pop() |
| 124 | try: |
| 125 | if exit(*exc): |
| 126 | exc = (None, None, None) |
| 127 | except: |
| 128 | exc = sys.exc_info() |
| 129 | if exc != (None, None, None): |
| 130 | # Don't rely on sys.exc_info() still containing |
| 131 | # the right information. Another exception may |
| 132 | # have been raised and caught by an exit method |
| 133 | raise exc[0], exc[1], exc[2] |
| 134 | |
| 135 | |
| 136 | class closing(object): |
| 137 | """Context to automatically close something at the end of a block. |
| 138 | |
| 139 | Code like this: |
| 140 | |
| 141 | with closing(<module>.open(<arguments>)) as f: |
| 142 | <block> |
| 143 | |
| 144 | is equivalent to this: |
| 145 | |
| 146 | f = <module>.open(<arguments>) |
| 147 | try: |
| 148 | <block> |
| 149 | finally: |
| 150 | f.close() |
| 151 | |
| 152 | """ |
| 153 | def __init__(self, thing): |
| 154 | self.thing = thing |
| 155 | def __enter__(self): |
| 156 | return self.thing |
| 157 | def __exit__(self, *exc_info): |
| 158 | self.thing.close() |