| """Utilities for with-statement contexts. See PEP 343.""" |
| |
| import sys |
| |
| __all__ = ["contextmanager", "nested", "closing"] |
| |
| class GeneratorContextManager(object): |
| """Helper for @contextmanager decorator.""" |
| |
| def __init__(self, gen): |
| self.gen = gen |
| |
| def __enter__(self): |
| try: |
| return self.gen.next() |
| except StopIteration: |
| raise RuntimeError("generator didn't yield") |
| |
| def __exit__(self, type, value, traceback): |
| if type is None: |
| try: |
| self.gen.next() |
| except StopIteration: |
| return |
| else: |
| raise RuntimeError("generator didn't stop") |
| else: |
| if value is None: |
| # Need to force instantiation so we can reliably |
| # tell if we get the same exception back |
| value = type() |
| try: |
| self.gen.throw(type, value, traceback) |
| raise RuntimeError("generator didn't stop after throw()") |
| except StopIteration, exc: |
| # Suppress the exception *unless* it's the same exception that |
| # was passed to throw(). This prevents a StopIteration |
| # raised inside the "with" statement from being suppressed |
| return exc is not value |
| except: |
| # only re-raise if it's *not* the exception that was |
| # passed to throw(), because __exit__() must not raise |
| # an exception unless __exit__() itself failed. But throw() |
| # has to raise the exception to signal propagation, so this |
| # fixes the impedance mismatch between the throw() protocol |
| # and the __exit__() protocol. |
| # |
| if sys.exc_info()[1] is not value: |
| raise |
| |
| |
| def contextmanager(func): |
| """@contextmanager decorator. |
| |
| Typical usage: |
| |
| @contextmanager |
| def some_generator(<arguments>): |
| <setup> |
| try: |
| yield <value> |
| finally: |
| <cleanup> |
| |
| This makes this: |
| |
| with some_generator(<arguments>) as <variable>: |
| <body> |
| |
| equivalent to this: |
| |
| <setup> |
| try: |
| <variable> = <value> |
| <body> |
| finally: |
| <cleanup> |
| |
| """ |
| def helper(*args, **kwds): |
| return GeneratorContextManager(func(*args, **kwds)) |
| try: |
| helper.__name__ = func.__name__ |
| helper.__doc__ = func.__doc__ |
| helper.__dict__ = func.__dict__ |
| except: |
| pass |
| return helper |
| |
| |
| @contextmanager |
| def nested(*managers): |
| """Support multiple context managers in a single with-statement. |
| |
| Code like this: |
| |
| with nested(A, B, C) as (X, Y, Z): |
| <body> |
| |
| is equivalent to this: |
| |
| with A as X: |
| with B as Y: |
| with C as Z: |
| <body> |
| |
| """ |
| exits = [] |
| vars = [] |
| exc = (None, None, None) |
| try: |
| try: |
| for mgr in managers: |
| exit = mgr.__exit__ |
| enter = mgr.__enter__ |
| vars.append(enter()) |
| exits.append(exit) |
| yield vars |
| except: |
| exc = sys.exc_info() |
| finally: |
| while exits: |
| exit = exits.pop() |
| try: |
| if exit(*exc): |
| exc = (None, None, None) |
| except: |
| exc = sys.exc_info() |
| if exc != (None, None, None): |
| # Don't rely on sys.exc_info() still containing |
| # the right information. Another exception may |
| # have been raised and caught by an exit method |
| raise exc[0], exc[1], exc[2] |
| |
| |
| class closing(object): |
| """Context to automatically close something at the end of a block. |
| |
| Code like this: |
| |
| with closing(<module>.open(<arguments>)) as f: |
| <block> |
| |
| is equivalent to this: |
| |
| f = <module>.open(<arguments>) |
| try: |
| <block> |
| finally: |
| f.close() |
| |
| """ |
| def __init__(self, thing): |
| self.thing = thing |
| def __enter__(self): |
| return self.thing |
| def __exit__(self, *exc_info): |
| self.thing.close() |
