"""Utilities for with-statement contexts. See PEP 343.""" import sys from collections import deque from functools import wraps __all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack"] 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 an otherwise one-shot context manager like _GeneratorContextManager to support use as a decorator via implicit recreation. This is a private interface just for _GeneratorContextManager. See issue #11647 for details. """ 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(): try: yield finally: This makes this: with some_generator() as : equivalent to this: try: = finally: """ @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(.open()) as f: is equivalent to this: f = .open() try: finally: f.close() """ def __init__(self, thing): self.thing = thing def __enter__(self): return self.thing def __exit__(self, *exc_info): self.thing.close() # Inspired by discussions on http://bugs.python.org/issue13585 class ExitStack(object): """Context manager for dynamic management of a stack of exit callbacks For example: with ExitStack() as stack: files = [stack.enter_context(open(fname)) for fname in filenames] # All opened files will automatically be closed at the end of # the with statement, even if attempts to open files later # in the list throw an exception """ def __init__(self): self._exit_callbacks = deque() def pop_all(self): """Preserve the context stack by transferring it to a new instance""" new_stack = type(self)() new_stack._exit_callbacks = self._exit_callbacks self._exit_callbacks = deque() return new_stack def _push_cm_exit(self, cm, cm_exit): """Helper to correctly register callbacks to __exit__ methods""" def _exit_wrapper(*exc_details): return cm_exit(cm, *exc_details) _exit_wrapper.__self__ = cm self.push(_exit_wrapper) def push(self, exit): """Registers a callback with the standard __exit__ method signature Can suppress exceptions the same way __exit__ methods can. Also accepts any object with an __exit__ method (registering a call to the method instead of the object itself) """ # We use an unbound method rather than a bound method to follow # the standard lookup behaviour for special methods _cb_type = type(exit) try: exit_method = _cb_type.__exit__ except AttributeError: # Not a context manager, so assume its a callable self._exit_callbacks.append(exit) else: self._push_cm_exit(exit, exit_method) return exit # Allow use as a decorator def callback(self, callback, *args, **kwds): """Registers an arbitrary callback and arguments. Cannot suppress exceptions. """ def _exit_wrapper(exc_type, exc, tb): callback(*args, **kwds) # We changed the signature, so using @wraps is not appropriate, but # setting __wrapped__ may still help with introspection _exit_wrapper.__wrapped__ = callback self.push(_exit_wrapper) return callback # Allow use as a decorator def enter_context(self, cm): """Enters the supplied context manager If successful, also pushes its __exit__ method as a callback and returns the result of the __enter__ method. """ # We look up the special methods on the type to match the with statement _cm_type = type(cm) _exit = _cm_type.__exit__ result = _cm_type.__enter__(cm) self._push_cm_exit(cm, _exit) return result def close(self): """Immediately unwind the context stack""" self.__exit__(None, None, None) def __enter__(self): return self def __exit__(self, *exc_details): if not self._exit_callbacks: return # This looks complicated, but it is really just # setting up a chain of try-expect statements to ensure # that outer callbacks still get invoked even if an # inner one throws an exception def _invoke_next_callback(exc_details): # Callbacks are removed from the list in FIFO order # but the recursion means they're invoked in LIFO order cb = self._exit_callbacks.popleft() if not self._exit_callbacks: # Innermost callback is invoked directly return cb(*exc_details) # More callbacks left, so descend another level in the stack try: suppress_exc = _invoke_next_callback(exc_details) except: suppress_exc = cb(*sys.exc_info()) # Check if this cb suppressed the inner exception if not suppress_exc: raise else: # Check if inner cb suppressed the original exception if suppress_exc: exc_details = (None, None, None) suppress_exc = cb(*exc_details) or suppress_exc return suppress_exc # Kick off the recursive chain return _invoke_next_callback(exc_details)