"""Utilities for with-statement contexts. See PEP 343.""" import sys from collections import deque from functools import wraps __all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack", "redirect_stdout", "suppress"] 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") from None 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 # Unfortunately, this was originally published as a class, so # backwards compatibility prevents the use of the wrapper function # approach used for the other classes 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() class _RedirectStdout: """Helper for redirect_stdout.""" def __init__(self, new_target): self._new_target = new_target self._old_target = self._sentinel = object() def __enter__(self): if self._old_target is not self._sentinel: raise RuntimeError("Cannot reenter {!r}".format(self)) self._old_target = sys.stdout sys.stdout = self._new_target return self._new_target def __exit__(self, exctype, excinst, exctb): restore_stdout = self._old_target self._old_target = self._sentinel sys.stdout = restore_stdout # Use a wrapper function since we don't care about supporting inheritance # and a function gives much cleaner output in help() def redirect_stdout(target): """Context manager for temporarily redirecting stdout to another file # How to send help() to stderr with redirect_stdout(sys.stderr): help(dir) # How to write help() to a file with open('help.txt', 'w') as f: with redirect_stdout(f): help(pow) """ return _RedirectStdout(target) class _SuppressExceptions: """Helper for suppress.""" def __init__(self, *exceptions): self._exceptions = exceptions def __enter__(self): pass def __exit__(self, exctype, excinst, exctb): # Unlike isinstance and issubclass, exception handling only # looks at the concrete type heirarchy (ignoring the instance # and subclass checking hooks). However, all exceptions are # also required to be concrete subclasses of BaseException, so # if there's a discrepancy in behaviour, we currently consider it # the fault of the strange way the exception has been defined rather # than the fact that issubclass can be customised while the # exception checks can't. # See http://bugs.python.org/issue12029 for more details return exctype is not None and issubclass(exctype, self._exceptions) # Use a wrapper function since we don't care about supporting inheritance # and a function gives much cleaner output in help() def suppress(*exceptions): """Context manager to suppress specified exceptions After the exception is suppressed, execution proceeds with the next statement following the with statement. with suppress(FileNotFoundError): os.remove(somefile) # Execution still resumes here if the file was already removed """ return _SuppressExceptions(*exceptions) # 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): while 1: exc_context = new_exc.__context__ if exc_context in (None, frame_exc): break new_exc = exc_context 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