| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 1 | """Utilities for with-statement contexts. See PEP 343.""" |
| 2 | |
| 3 | import sys |
| Christian Heimes | 81ee3ef | 2008-05-04 22:42:01 +0000 | [diff] [blame] | 4 | from functools import wraps |
| Raymond Hettinger | 91e3b9d | 2009-05-28 22:20:03 +0000 | [diff] [blame] | 5 | from warnings import warn |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 6 | |
| Benjamin Peterson | 04e40c1 | 2009-07-01 01:38:23 +0000 | [diff] [blame] | 7 | __all__ = ["contextmanager", "closing"] |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 8 | |
| 9 | class GeneratorContextManager(object): |
| 10 | """Helper for @contextmanager decorator.""" |
| 11 | |
| 12 | def __init__(self, gen): |
| 13 | self.gen = gen |
| 14 | |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 15 | def __enter__(self): |
| 16 | try: |
| Georg Brandl | a18af4e | 2007-04-21 15:47:16 +0000 | [diff] [blame] | 17 | return next(self.gen) |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 18 | except StopIteration: |
| 19 | raise RuntimeError("generator didn't yield") |
| 20 | |
| 21 | def __exit__(self, type, value, traceback): |
| 22 | if type is None: |
| 23 | try: |
| Georg Brandl | a18af4e | 2007-04-21 15:47:16 +0000 | [diff] [blame] | 24 | next(self.gen) |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 25 | except StopIteration: |
| 26 | return |
| 27 | else: |
| 28 | raise RuntimeError("generator didn't stop") |
| 29 | else: |
| Guido van Rossum | 2cc30da | 2007-11-02 23:46:40 +0000 | [diff] [blame] | 30 | if value is None: |
| 31 | # Need to force instantiation so we can reliably |
| 32 | # tell if we get the same exception back |
| 33 | value = type() |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 34 | try: |
| 35 | self.gen.throw(type, value, traceback) |
| Thomas Wouters | 49fd7fa | 2006-04-21 10:40:58 +0000 | [diff] [blame] | 36 | raise RuntimeError("generator didn't stop after throw()") |
| Guido van Rossum | b940e11 | 2007-01-10 16:19:56 +0000 | [diff] [blame] | 37 | except StopIteration as exc: |
| Thomas Wouters | 49fd7fa | 2006-04-21 10:40:58 +0000 | [diff] [blame] | 38 | # Suppress the exception *unless* it's the same exception that |
| 39 | # was passed to throw(). This prevents a StopIteration |
| 40 | # raised inside the "with" statement from being suppressed |
| 41 | return exc is not value |
| 42 | except: |
| 43 | # only re-raise if it's *not* the exception that was |
| 44 | # passed to throw(), because __exit__() must not raise |
| 45 | # an exception unless __exit__() itself failed. But throw() |
| 46 | # has to raise the exception to signal propagation, so this |
| 47 | # fixes the impedance mismatch between the throw() protocol |
| 48 | # and the __exit__() protocol. |
| 49 | # |
| 50 | if sys.exc_info()[1] is not value: |
| 51 | raise |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 52 | |
| 53 | |
| 54 | def contextmanager(func): |
| 55 | """@contextmanager decorator. |
| 56 | |
| 57 | Typical usage: |
| 58 | |
| 59 | @contextmanager |
| 60 | def some_generator(<arguments>): |
| 61 | <setup> |
| 62 | try: |
| 63 | yield <value> |
| 64 | finally: |
| 65 | <cleanup> |
| 66 | |
| 67 | This makes this: |
| 68 | |
| 69 | with some_generator(<arguments>) as <variable>: |
| 70 | <body> |
| 71 | |
| 72 | equivalent to this: |
| 73 | |
| 74 | <setup> |
| 75 | try: |
| 76 | <variable> = <value> |
| 77 | <body> |
| 78 | finally: |
| 79 | <cleanup> |
| 80 | |
| 81 | """ |
| Christian Heimes | 81ee3ef | 2008-05-04 22:42:01 +0000 | [diff] [blame] | 82 | @wraps(func) |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 83 | def helper(*args, **kwds): |
| 84 | return GeneratorContextManager(func(*args, **kwds)) |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 85 | return helper |
| 86 | |
| 87 | |
| Thomas Wouters | 477c8d5 | 2006-05-27 19:21:47 +0000 | [diff] [blame] | 88 | class closing(object): |
| 89 | """Context to automatically close something at the end of a block. |
| Guido van Rossum | 1a5e21e | 2006-02-28 21:57:43 +0000 | [diff] [blame] | 90 | |
| 91 | Code like this: |
| 92 | |
| 93 | with closing(<module>.open(<arguments>)) as f: |
| 94 | <block> |
| 95 | |
| 96 | is equivalent to this: |
| 97 | |
| 98 | f = <module>.open(<arguments>) |
| 99 | try: |
| 100 | <block> |
| 101 | finally: |
| 102 | f.close() |
| 103 | |
| 104 | """ |
| Thomas Wouters | 477c8d5 | 2006-05-27 19:21:47 +0000 | [diff] [blame] | 105 | def __init__(self, thing): |
| 106 | self.thing = thing |
| 107 | def __enter__(self): |
| 108 | return self.thing |
| 109 | def __exit__(self, *exc_info): |
| 110 | self.thing.close() |