| """Utilities for with-statement contexts. See PEP 343.""" |
| |
| import sys |
| from functools import wraps |
| from warnings import warn |
| |
| __all__ = ["contextmanager", "closing", "ContextDecorator"] |
| |
| |
| class ContextDecorator(object): |
| "A base class or mixin that enables context managers to work as decorators." |
| |
| def _recreate_cm(self): |
| """Return a recreated instance of self. |
| |
| Allows otherwise one-shot context managers like |
| _GeneratorContextManager to support use as |
| decorators via implicit recreation. |
| |
| Note: this is a private interface just for _GCM in 3.2 but will be |
| renamed and documented for third party use in 3.3 |
| """ |
| return self |
| |
| def __call__(self, func): |
| @wraps(func) |
| def inner(*args, **kwds): |
| with self._recreate_cm(): |
| return func(*args, **kwds) |
| return inner |
| |
| |
| class _GeneratorContextManager(ContextDecorator): |
| """Helper for @contextmanager decorator.""" |
| |
| def __init__(self, func, *args, **kwds): |
| self.gen = func(*args, **kwds) |
| self.func, self.args, self.kwds = func, args, kwds |
| |
| def _recreate_cm(self): |
| # _GCM instances are one-shot context managers, so the |
| # CM must be recreated each time a decorated function is |
| # called |
| return self.__class__(self.func, *self.args, **self.kwds) |
| |
| def __enter__(self): |
| try: |
| return next(self.gen) |
| except StopIteration: |
| raise RuntimeError("generator didn't yield") |
| |
| def __exit__(self, type, value, traceback): |
| if type is None: |
| try: |
| next(self.gen) |
| 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 as 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> |
| |
| """ |
| @wraps(func) |
| def helper(*args, **kwds): |
| return _GeneratorContextManager(func, *args, **kwds) |
| return helper |
| |
| |
| 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() |