"""Utilities for with-statement contexts. See PEP 343. Original source code: https://hg.python.org/cpython/file/3.4/Lib/contextlib.py Not implemented: - redirect_stdout; - 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): 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 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: """ 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() class suppress: """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 """ def __init__(self, *exceptions): self._exceptions = exceptions def __enter__(self): pass def __exit__(self, exctype, excinst, exctb): # Unlike isinstance and issubclass, CPython exception handling # currently only looks at the concrete type hierarchy (ignoring # the instance and subclass checking hooks). While Guido considers # that a bug rather than a feature, it's a fairly hard one to fix # due to various internal implementation details. suppress provides # the simpler issubclass based semantics, rather than trying to # exactly reproduce the limitations of the CPython interpreter. # # See http://bugs.python.org/issue12029 for more details return exctype is not None and issubclass(exctype, self._exceptions)