diff options
Diffstat (limited to 'Lib/contextlib.py')
| -rw-r--r-- | Lib/contextlib.py | 144 |
1 files changed, 138 insertions, 6 deletions
diff --git a/Lib/contextlib.py b/Lib/contextlib.py index 5ebbbc6583..b03f828319 100644 --- a/Lib/contextlib.py +++ b/Lib/contextlib.py @@ -1,10 +1,10 @@ """Utilities for with-statement contexts. See PEP 343.""" import sys +from collections import deque from functools import wraps -from warnings import warn -__all__ = ["contextmanager", "closing", "ContextDecorator"] +__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack"] class ContextDecorator(object): @@ -13,12 +13,12 @@ class ContextDecorator(object): def _recreate_cm(self): """Return a recreated instance of self. - Allows otherwise one-shot context managers like + Allows an otherwise one-shot context manager like _GeneratorContextManager to support use as - decorators via implicit recreation. + a decorator 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 + This is a private interface just for _GeneratorContextManager. + See issue #11647 for details. """ return self @@ -139,3 +139,135 @@ class closing(object): 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 raise 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): + received_exc = exc_details[0] is not None + + # We manipulate the exception state so it behaves as though + # we were actually nesting multiple with statements + frame_exc = sys.exc_info()[1] + def _fix_exception_context(new_exc, old_exc): + # Context may not be correct, so find the end of the chain + while 1: + exc_context = new_exc.__context__ + if exc_context is old_exc: + # Context is already set correctly (see issue 20317) + return + if exc_context is None or exc_context is frame_exc: + break + new_exc = exc_context + # Change the end of the chain to point to the exception + # we expect it to reference + new_exc.__context__ = old_exc + + # Callbacks are invoked in LIFO order to match the behaviour of + # nested context managers + suppressed_exc = False + pending_raise = False + while self._exit_callbacks: + cb = self._exit_callbacks.pop() + try: + if cb(*exc_details): + suppressed_exc = True + pending_raise = False + exc_details = (None, None, None) + except: + new_exc_details = sys.exc_info() + # simulate the stack of exceptions by setting the context + _fix_exception_context(new_exc_details[1], exc_details[1]) + pending_raise = True + exc_details = new_exc_details + if pending_raise: + try: + # bare "raise exc_details[1]" replaces our carefully + # set-up context + fixed_ctx = exc_details[1].__context__ + raise exc_details[1] + except BaseException: + exc_details[1].__context__ = fixed_ctx + raise + return received_exc and suppressed_exc |