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Diffstat (limited to 'modules/language/python/module/contextlib.py')
| -rw-r--r-- | modules/language/python/module/contextlib.py | 386 |
1 files changed, 386 insertions, 0 deletions
diff --git a/modules/language/python/module/contextlib.py b/modules/language/python/module/contextlib.py new file mode 100644 index 0000000..739cc28 --- /dev/null +++ b/modules/language/python/module/contextlib.py @@ -0,0 +1,386 @@ +module(contextlib) + +"""Utilities for with-statement contexts. See PEP 343.""" +import abc +import sys +import collections.abc +from collections import deque +from functools import wraps + +__all__ = ["contextmanager", "closing", "AbstractContextManager", + "ContextDecorator", "ExitStack", "redirect_stdout", + "redirect_stderr", "suppress"] + + +class AbstractContextManager(abc.ABC): + + """An abstract base class for context managers.""" + + def __enter__(self): + """Return `self` upon entering the runtime context.""" + return self + + @abc.abstractmethod + def __exit__(self, exc_type, exc_value, traceback): + """Raise any exception triggered within the runtime context.""" + return None + + @classmethod + def __subclasshook__(cls, C): + if cls is AbstractContextManager: + return collections.abc._check_methods(C, "__enter__", "__exit__") + return NotImplemented + + +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, AbstractContextManager): + """Helper for @contextmanager decorator.""" + + def __init__(self, func, args, kwds): + self.gen = func(*args, **kwds) + self.func, self.args, self.kwds = func, args, kwds + # Issue 19330: ensure context manager instances have good docstrings + doc = getattr(func, "__doc__", None) + if doc is None: + doc = type(self).__doc__ + self.__doc__ = doc + # Unfortunately, this still doesn't provide good help output when + # inspecting the created context manager instances, since pydoc + # currently bypasses the instance docstring and shows the docstring + # for the class instead. + # See http://bugs.python.org/issue19404 for more details. + + 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 False + 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) + except StopIteration as exc: + # Suppress StopIteration *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 RuntimeError as exc: + # Don't re-raise the passed in exception. (issue27122) + if exc is value: + return False + # Likewise, avoid suppressing if a StopIteration exception + # was passed to throw() and later wrapped into a RuntimeError + # (see PEP 479). + if type is StopIteration and exc.__cause__ is value: + return False + raise + 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 value: + return False + raise + raise RuntimeError("generator didn't stop after throw()") + + +def contextmanager(func): + """@contextmanager decorator. + + Typical usage: + + @contextmanager + def some_generator(<arguments>): + <setup> + try: + yield <value> + finally: + <cleanup> + + This makes this: + + with some_generator(<arguments>) as <variable>: + <body> + + equivalent to this: + + <setup> + try: + <variable> = <value> + <body> + finally: + <cleanup> + + """ + @wraps(func) + def helper(*args, **kwds): + return _GeneratorContextManager(func, args, kwds) + return helper + + +class closing(AbstractContextManager): + """Context to automatically close something at the end of a block. + + Code like this: + + with closing(<module>.open(<arguments>)) as f: + <block> + + is equivalent to this: + + f = <module>.open(<arguments>) + try: + <block> + 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 _RedirectStream(AbstractContextManager): + + _stream = None + + def __init__(self, new_target): + self._new_target = new_target + # We use a list of old targets to make this CM re-entrant + self._old_targets = [] + + def __enter__(self): + self._old_targets.append(getattr(sys, self._stream)) + setattr(sys, self._stream, self._new_target) + return self._new_target + + def __exit__(self, exctype, excinst, exctb): + setattr(sys, self._stream, self._old_targets.pop()) + + +class redirect_stdout(_RedirectStream): + """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) + """ + + _stream = "stdout" + + +class redirect_stderr(_RedirectStream): + """Context manager for temporarily redirecting stderr to another file.""" + + _stream = "stderr" + + +class suppress(AbstractContextManager): + """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) + + +# Inspired by discussions on http://bugs.python.org/issue13585 +class ExitStack(AbstractContextManager): + """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 __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 |