2011-12-13 12:41:17 +00:00
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"""contextlib2 - backports and enhancements to the contextlib module"""
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import sys
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from collections import deque
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from functools import wraps
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2011-12-15 11:07:03 +00:00
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__all__ = ["contextmanager", "closing", "ContextDecorator", "ContextStack"]
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2011-12-13 12:41:17 +00:00
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class ContextDecorator(object):
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"A base class or mixin that enables context managers to work as decorators."
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def refresh_cm(self):
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"""Returns the context manager used to actually wrap the call to the
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decorated function.
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The default implementation just returns *self*.
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Overriding this method allows otherwise one-shot context managers
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like _GeneratorContextManager to support use as decorators via
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implicit recreation.
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"""
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return self
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def __call__(self, func):
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@wraps(func)
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def inner(*args, **kwds):
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with self.refresh_cm():
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return func(*args, **kwds)
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return inner
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class _GeneratorContextManager(ContextDecorator):
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"""Helper for @contextmanager decorator."""
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def __init__(self, func, *args, **kwds):
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self.gen = func(*args, **kwds)
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self.func, self.args, self.kwds = func, args, kwds
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def refresh_cm(self):
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# _GCM instances are one-shot context managers, so the
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# CM must be recreated each time a decorated function is
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# called
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return self.__class__(self.func, *self.args, **self.kwds)
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def __enter__(self):
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try:
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return next(self.gen)
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except StopIteration:
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raise RuntimeError("generator didn't yield")
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def __exit__(self, type, value, traceback):
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if type is None:
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try:
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next(self.gen)
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except StopIteration:
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return
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else:
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raise RuntimeError("generator didn't stop")
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else:
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if value is None:
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# Need to force instantiation so we can reliably
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# tell if we get the same exception back
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value = type()
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try:
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self.gen.throw(type, value, traceback)
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raise RuntimeError("generator didn't stop after throw()")
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except StopIteration as exc:
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# Suppress the exception *unless* it's the same exception that
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# was passed to throw(). This prevents a StopIteration
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# raised inside the "with" statement from being suppressed
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return exc is not value
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except:
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# only re-raise if it's *not* the exception that was
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# passed to throw(), because __exit__() must not raise
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# an exception unless __exit__() itself failed. But throw()
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# has to raise the exception to signal propagation, so this
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# fixes the impedance mismatch between the throw() protocol
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# and the __exit__() protocol.
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#
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if sys.exc_info()[1] is not value:
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raise
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def contextmanager(func):
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"""@contextmanager decorator.
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Typical usage:
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@contextmanager
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def some_generator(<arguments>):
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<setup>
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try:
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yield <value>
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finally:
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<cleanup>
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This makes this:
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with some_generator(<arguments>) as <variable>:
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<body>
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equivalent to this:
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<setup>
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try:
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<variable> = <value>
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<body>
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finally:
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<cleanup>
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"""
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@wraps(func)
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def helper(*args, **kwds):
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return _GeneratorContextManager(func, *args, **kwds)
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return helper
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class closing(object):
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"""Context to automatically close something at the end of a block.
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Code like this:
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with closing(<module>.open(<arguments>)) as f:
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<block>
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is equivalent to this:
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f = <module>.open(<arguments>)
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try:
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<block>
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finally:
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f.close()
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"""
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def __init__(self, thing):
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self.thing = thing
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def __enter__(self):
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return self.thing
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def __exit__(self, *exc_info):
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self.thing.close()
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2011-12-15 11:07:03 +00:00
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class ContextStack(object):
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2011-12-13 12:41:17 +00:00
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"""Context for programmatic management of resource cleanup
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For example:
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2011-12-15 11:07:03 +00:00
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with ContextStack() as stack:
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files = [stack.enter_context(fname) for fname in filenames]
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2011-12-13 12:41:17 +00:00
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# All opened files will automatically be closed at the end of
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# the with statement, even if attempts to open files later
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# in the list throw an exception
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"""
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def __init__(self):
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self._callbacks = deque()
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2011-12-15 11:07:03 +00:00
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def register_exit(self, callback):
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"""Registers a callback with the standard __exit__ method signature
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2011-12-13 12:41:17 +00:00
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2011-12-15 11:07:03 +00:00
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Can suppress exceptions the same way __exit__ methods can.
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2011-12-13 12:41:17 +00:00
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"""
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2011-12-15 11:07:03 +00:00
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self._callbacks.append(callback)
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return callback # Allow use as a decorator
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2011-12-13 12:41:17 +00:00
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2011-12-15 11:07:03 +00:00
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def register(self, callback, *args, **kwds):
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"""Registers an arbitrary callback and arguments.
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Cannot suppress exceptions.
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"""
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def _wrapper(exc_type, exc, tb):
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callback(*args, **kwds)
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2011-12-13 12:41:17 +00:00
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return self.register_exit(_wrapper)
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def enter_context(self, cm):
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"""Enters the supplied context manager
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If successful, also registers its __exit__ method as a callback and
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returns the result of the __enter__ method.
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"""
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2011-12-13 12:41:17 +00:00
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# We look up the special methods on the type to match the with statement
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_cm_type = type(cm)
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_exit = _cm_type.__exit__
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result = _cm_type.__enter__(cm)
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def _exit_wrapper(*exc_details):
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return _exit(cm, *exc_details)
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self.register_exit(_exit_wrapper)
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return result
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def close(self):
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"""Immediately unwind the context stack"""
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2011-12-13 12:41:17 +00:00
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self.__exit__(None, None, None)
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def __enter__(self):
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return self
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def __exit__(self, *exc_details):
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if not self._callbacks:
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return
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# This looks complicated, but it is really just
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# setting up a chain of try-expect statements to ensure
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# that outer callbacks still get invoked even if an
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# inner one throws an exception
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def _invoke_next_callback(exc_details):
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# Callbacks are removed from the list in FIFO order
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# but the recursion means they're invoked in LIFO order
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cb = self._callbacks.popleft()
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if not self._callbacks:
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# Innermost callback is invoked directly
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return cb(*exc_details)
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# More callbacks left, so descend another level in the stack
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try:
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suppress_exc = _invoke_next_callback(exc_details)
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except:
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suppress_exc = cb(*sys.exc_info())
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# Check if this cb suppressed the inner exception
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if not suppress_exc:
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raise
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else:
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# Check if inner cb suppressed the original exception
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if suppress_exc:
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exc_details = (None, None, None)
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suppress_exc = cb(*exc_details) or suppress_exc
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return suppress_exc
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# Kick off the recursive chain
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return _invoke_next_callback(exc_details)
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