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678 lines
23 KiB
678 lines
23 KiB
6 years ago
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"""Utilities for with-statement contexts. See PEP 343."""
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import abc
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import sys
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import _collections_abc
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from collections import deque
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from functools import wraps
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__all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
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"AbstractContextManager", "AbstractAsyncContextManager",
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"AsyncExitStack", "ContextDecorator", "ExitStack",
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"redirect_stdout", "redirect_stderr", "suppress"]
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class AbstractContextManager(abc.ABC):
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"""An abstract base class for context managers."""
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def __enter__(self):
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"""Return `self` upon entering the runtime context."""
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return self
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@abc.abstractmethod
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def __exit__(self, exc_type, exc_value, traceback):
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"""Raise any exception triggered within the runtime context."""
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return None
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@classmethod
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def __subclasshook__(cls, C):
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if cls is AbstractContextManager:
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return _collections_abc._check_methods(C, "__enter__", "__exit__")
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return NotImplemented
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class AbstractAsyncContextManager(abc.ABC):
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"""An abstract base class for asynchronous context managers."""
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async def __aenter__(self):
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"""Return `self` upon entering the runtime context."""
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return self
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@abc.abstractmethod
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async def __aexit__(self, exc_type, exc_value, traceback):
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"""Raise any exception triggered within the runtime context."""
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return None
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@classmethod
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def __subclasshook__(cls, C):
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if cls is AbstractAsyncContextManager:
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return _collections_abc._check_methods(C, "__aenter__",
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"__aexit__")
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return NotImplemented
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class ContextDecorator(object):
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"A base class or mixin that enables context managers to work as decorators."
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def _recreate_cm(self):
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"""Return a recreated instance of self.
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Allows an otherwise one-shot context manager like
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_GeneratorContextManager to support use as
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a decorator via implicit recreation.
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This is a private interface just for _GeneratorContextManager.
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See issue #11647 for details.
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"""
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return self
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def __call__(self, func):
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@wraps(func)
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def inner(*args, **kwds):
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with self._recreate_cm():
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return func(*args, **kwds)
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return inner
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class _GeneratorContextManagerBase:
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"""Shared functionality for @contextmanager and @asynccontextmanager."""
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def __init__(self, func, args, kwds):
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self.gen = func(*args, **kwds)
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self.func, self.args, self.kwds = func, args, kwds
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# Issue 19330: ensure context manager instances have good docstrings
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doc = getattr(func, "__doc__", None)
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if doc is None:
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doc = type(self).__doc__
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self.__doc__ = doc
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# Unfortunately, this still doesn't provide good help output when
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# inspecting the created context manager instances, since pydoc
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# currently bypasses the instance docstring and shows the docstring
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# for the class instead.
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# See http://bugs.python.org/issue19404 for more details.
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class _GeneratorContextManager(_GeneratorContextManagerBase,
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AbstractContextManager,
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ContextDecorator):
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"""Helper for @contextmanager decorator."""
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def _recreate_cm(self):
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# _GCM instances are one-shot context managers, so the
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# CM must be recreated each time a decorated function is
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# called
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return self.__class__(self.func, self.args, self.kwds)
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def __enter__(self):
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# do not keep args and kwds alive unnecessarily
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# they are only needed for recreation, which is not possible anymore
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del self.args, self.kwds, self.func
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try:
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return next(self.gen)
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except StopIteration:
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raise RuntimeError("generator didn't yield") from None
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def __exit__(self, type, value, traceback):
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if type is None:
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try:
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next(self.gen)
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except StopIteration:
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return False
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else:
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raise RuntimeError("generator didn't stop")
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else:
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if value is None:
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# Need to force instantiation so we can reliably
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# tell if we get the same exception back
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value = type()
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try:
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self.gen.throw(type, value, traceback)
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except StopIteration as exc:
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# Suppress StopIteration *unless* it's the same exception that
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# was passed to throw(). This prevents a StopIteration
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# raised inside the "with" statement from being suppressed.
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return exc is not value
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except RuntimeError as exc:
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# Don't re-raise the passed in exception. (issue27122)
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if exc is value:
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return False
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# Likewise, avoid suppressing if a StopIteration exception
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# was passed to throw() and later wrapped into a RuntimeError
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# (see PEP 479).
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if type is StopIteration and exc.__cause__ is value:
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return False
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raise
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except:
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# only re-raise if it's *not* the exception that was
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# passed to throw(), because __exit__() must not raise
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# an exception unless __exit__() itself failed. But throw()
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# has to raise the exception to signal propagation, so this
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# fixes the impedance mismatch between the throw() protocol
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# and the __exit__() protocol.
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#
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# This cannot use 'except BaseException as exc' (as in the
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# async implementation) to maintain compatibility with
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# Python 2, where old-style class exceptions are not caught
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# by 'except BaseException'.
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if sys.exc_info()[1] is value:
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return False
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raise
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raise RuntimeError("generator didn't stop after throw()")
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class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
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AbstractAsyncContextManager):
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"""Helper for @asynccontextmanager."""
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async def __aenter__(self):
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try:
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return await self.gen.__anext__()
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except StopAsyncIteration:
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raise RuntimeError("generator didn't yield") from None
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async def __aexit__(self, typ, value, traceback):
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if typ is None:
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try:
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await self.gen.__anext__()
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except StopAsyncIteration:
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return
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else:
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raise RuntimeError("generator didn't stop")
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else:
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if value is None:
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value = typ()
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# See _GeneratorContextManager.__exit__ for comments on subtleties
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# in this implementation
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try:
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await self.gen.athrow(typ, value, traceback)
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raise RuntimeError("generator didn't stop after throw()")
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except StopAsyncIteration as exc:
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return exc is not value
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except RuntimeError as exc:
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if exc is value:
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return False
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# Avoid suppressing if a StopIteration exception
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# was passed to throw() and later wrapped into a RuntimeError
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# (see PEP 479 for sync generators; async generators also
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# have this behavior). But do this only if the exception wrapped
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# by the RuntimeError is actully Stop(Async)Iteration (see
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# issue29692).
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if isinstance(value, (StopIteration, StopAsyncIteration)):
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if exc.__cause__ is value:
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return False
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raise
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except BaseException as exc:
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if exc is not value:
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raise
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def contextmanager(func):
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"""@contextmanager decorator.
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Typical usage:
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@contextmanager
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def some_generator(<arguments>):
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<setup>
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try:
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yield <value>
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finally:
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<cleanup>
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This makes this:
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with some_generator(<arguments>) as <variable>:
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<body>
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equivalent to this:
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<setup>
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try:
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<variable> = <value>
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<body>
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finally:
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<cleanup>
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"""
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@wraps(func)
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def helper(*args, **kwds):
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return _GeneratorContextManager(func, args, kwds)
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return helper
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def asynccontextmanager(func):
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"""@asynccontextmanager decorator.
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Typical usage:
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@asynccontextmanager
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async def some_async_generator(<arguments>):
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<setup>
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try:
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yield <value>
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finally:
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<cleanup>
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This makes this:
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async with some_async_generator(<arguments>) as <variable>:
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<body>
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equivalent to this:
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<setup>
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try:
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<variable> = <value>
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<body>
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finally:
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<cleanup>
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"""
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@wraps(func)
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def helper(*args, **kwds):
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return _AsyncGeneratorContextManager(func, args, kwds)
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return helper
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class closing(AbstractContextManager):
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"""Context to automatically close something at the end of a block.
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Code like this:
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with closing(<module>.open(<arguments>)) as f:
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<block>
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is equivalent to this:
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f = <module>.open(<arguments>)
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try:
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<block>
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finally:
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f.close()
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"""
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def __init__(self, thing):
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self.thing = thing
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def __enter__(self):
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return self.thing
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def __exit__(self, *exc_info):
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self.thing.close()
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class _RedirectStream(AbstractContextManager):
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_stream = None
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def __init__(self, new_target):
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self._new_target = new_target
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# We use a list of old targets to make this CM re-entrant
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self._old_targets = []
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def __enter__(self):
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self._old_targets.append(getattr(sys, self._stream))
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setattr(sys, self._stream, self._new_target)
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return self._new_target
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def __exit__(self, exctype, excinst, exctb):
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setattr(sys, self._stream, self._old_targets.pop())
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class redirect_stdout(_RedirectStream):
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"""Context manager for temporarily redirecting stdout to another file.
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# How to send help() to stderr
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with redirect_stdout(sys.stderr):
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help(dir)
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# How to write help() to a file
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with open('help.txt', 'w') as f:
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with redirect_stdout(f):
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help(pow)
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"""
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_stream = "stdout"
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class redirect_stderr(_RedirectStream):
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"""Context manager for temporarily redirecting stderr to another file."""
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_stream = "stderr"
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class suppress(AbstractContextManager):
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"""Context manager to suppress specified exceptions
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After the exception is suppressed, execution proceeds with the next
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statement following the with statement.
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with suppress(FileNotFoundError):
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os.remove(somefile)
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# Execution still resumes here if the file was already removed
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"""
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def __init__(self, *exceptions):
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self._exceptions = exceptions
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def __enter__(self):
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pass
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def __exit__(self, exctype, excinst, exctb):
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# Unlike isinstance and issubclass, CPython exception handling
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# currently only looks at the concrete type hierarchy (ignoring
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# the instance and subclass checking hooks). While Guido considers
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# that a bug rather than a feature, it's a fairly hard one to fix
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# due to various internal implementation details. suppress provides
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# the simpler issubclass based semantics, rather than trying to
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# exactly reproduce the limitations of the CPython interpreter.
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#
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# See http://bugs.python.org/issue12029 for more details
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return exctype is not None and issubclass(exctype, self._exceptions)
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class _BaseExitStack:
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"""A base class for ExitStack and AsyncExitStack."""
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@staticmethod
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def _create_exit_wrapper(cm, cm_exit):
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def _exit_wrapper(exc_type, exc, tb):
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return cm_exit(cm, exc_type, exc, tb)
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return _exit_wrapper
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@staticmethod
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def _create_cb_wrapper(callback, *args, **kwds):
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def _exit_wrapper(exc_type, exc, tb):
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callback(*args, **kwds)
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return _exit_wrapper
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def __init__(self):
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self._exit_callbacks = deque()
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def pop_all(self):
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"""Preserve the context stack by transferring it to a new instance."""
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new_stack = type(self)()
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new_stack._exit_callbacks = self._exit_callbacks
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self._exit_callbacks = deque()
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return new_stack
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def push(self, exit):
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"""Registers a callback with the standard __exit__ method signature.
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Can suppress exceptions the same way __exit__ method can.
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Also accepts any object with an __exit__ method (registering a call
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to the method instead of the object itself).
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"""
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# We use an unbound method rather than a bound method to follow
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# the standard lookup behaviour for special methods.
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_cb_type = type(exit)
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try:
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|
|
exit_method = _cb_type.__exit__
|
||
|
|
except AttributeError:
|
||
|
|
# Not a context manager, so assume it's a callable.
|
||
|
|
self._push_exit_callback(exit)
|
||
|
|
else:
|
||
|
|
self._push_cm_exit(exit, exit_method)
|
||
|
|
return exit # 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 callback(self, callback, *args, **kwds):
|
||
|
|
"""Registers an arbitrary callback and arguments.
|
||
|
|
|
||
|
|
Cannot suppress exceptions.
|
||
|
|
"""
|
||
|
|
_exit_wrapper = self._create_cb_wrapper(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_callback(_exit_wrapper)
|
||
|
|
return callback # Allow use as a decorator
|
||
|
|
|
||
|
|
def _push_cm_exit(self, cm, cm_exit):
|
||
|
|
"""Helper to correctly register callbacks to __exit__ methods."""
|
||
|
|
_exit_wrapper = self._create_exit_wrapper(cm, cm_exit)
|
||
|
|
_exit_wrapper.__self__ = cm
|
||
|
|
self._push_exit_callback(_exit_wrapper, True)
|
||
|
|
|
||
|
|
def _push_exit_callback(self, callback, is_sync=True):
|
||
|
|
self._exit_callbacks.append((is_sync, callback))
|
||
|
|
|
||
|
|
|
||
|
|
# Inspired by discussions on http://bugs.python.org/issue13585
|
||
|
|
class ExitStack(_BaseExitStack, 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 __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:
|
||
|
|
is_sync, cb = self._exit_callbacks.pop()
|
||
|
|
assert is_sync
|
||
|
|
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
|
||
|
|
|
||
|
|
def close(self):
|
||
|
|
"""Immediately unwind the context stack."""
|
||
|
|
self.__exit__(None, None, None)
|
||
|
|
|
||
|
|
|
||
|
|
# Inspired by discussions on https://bugs.python.org/issue29302
|
||
|
|
class AsyncExitStack(_BaseExitStack, AbstractAsyncContextManager):
|
||
|
|
"""Async context manager for dynamic management of a stack of exit
|
||
|
|
callbacks.
|
||
|
|
|
||
|
|
For example:
|
||
|
|
async with AsyncExitStack() as stack:
|
||
|
|
connections = [await stack.enter_async_context(get_connection())
|
||
|
|
for i in range(5)]
|
||
|
|
# All opened connections will automatically be released at the
|
||
|
|
# end of the async with statement, even if attempts to open a
|
||
|
|
# connection later in the list raise an exception.
|
||
|
|
"""
|
||
|
|
|
||
|
|
@staticmethod
|
||
|
|
def _create_async_exit_wrapper(cm, cm_exit):
|
||
|
|
async def _exit_wrapper(exc_type, exc, tb):
|
||
|
|
return await cm_exit(cm, exc_type, exc, tb)
|
||
|
|
return _exit_wrapper
|
||
|
|
|
||
|
|
@staticmethod
|
||
|
|
def _create_async_cb_wrapper(callback, *args, **kwds):
|
||
|
|
async def _exit_wrapper(exc_type, exc, tb):
|
||
|
|
await callback(*args, **kwds)
|
||
|
|
return _exit_wrapper
|
||
|
|
|
||
|
|
async def enter_async_context(self, cm):
|
||
|
|
"""Enters the supplied async context manager.
|
||
|
|
|
||
|
|
If successful, also pushes its __aexit__ method as a callback and
|
||
|
|
returns the result of the __aenter__ method.
|
||
|
|
"""
|
||
|
|
_cm_type = type(cm)
|
||
|
|
_exit = _cm_type.__aexit__
|
||
|
|
result = await _cm_type.__aenter__(cm)
|
||
|
|
self._push_async_cm_exit(cm, _exit)
|
||
|
|
return result
|
||
|
|
|
||
|
|
def push_async_exit(self, exit):
|
||
|
|
"""Registers a coroutine function with the standard __aexit__ method
|
||
|
|
signature.
|
||
|
|
|
||
|
|
Can suppress exceptions the same way __aexit__ method can.
|
||
|
|
Also accepts any object with an __aexit__ method (registering a call
|
||
|
|
to the method instead of the object itself).
|
||
|
|
"""
|
||
|
|
_cb_type = type(exit)
|
||
|
|
try:
|
||
|
|
exit_method = _cb_type.__aexit__
|
||
|
|
except AttributeError:
|
||
|
|
# Not an async context manager, so assume it's a coroutine function
|
||
|
|
self._push_exit_callback(exit, False)
|
||
|
|
else:
|
||
|
|
self._push_async_cm_exit(exit, exit_method)
|
||
|
|
return exit # Allow use as a decorator
|
||
|
|
|
||
|
|
def push_async_callback(self, callback, *args, **kwds):
|
||
|
|
"""Registers an arbitrary coroutine function and arguments.
|
||
|
|
|
||
|
|
Cannot suppress exceptions.
|
||
|
|
"""
|
||
|
|
_exit_wrapper = self._create_async_cb_wrapper(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_callback(_exit_wrapper, False)
|
||
|
|
return callback # Allow use as a decorator
|
||
|
|
|
||
|
|
async def aclose(self):
|
||
|
|
"""Immediately unwind the context stack."""
|
||
|
|
await self.__aexit__(None, None, None)
|
||
|
|
|
||
|
|
def _push_async_cm_exit(self, cm, cm_exit):
|
||
|
|
"""Helper to correctly register coroutine function to __aexit__
|
||
|
|
method."""
|
||
|
|
_exit_wrapper = self._create_async_exit_wrapper(cm, cm_exit)
|
||
|
|
_exit_wrapper.__self__ = cm
|
||
|
|
self._push_exit_callback(_exit_wrapper, False)
|
||
|
|
|
||
|
|
async def __aenter__(self):
|
||
|
|
return self
|
||
|
|
|
||
|
|
async def __aexit__(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:
|
||
|
|
is_sync, cb = self._exit_callbacks.pop()
|
||
|
|
try:
|
||
|
|
if is_sync:
|
||
|
|
cb_suppress = cb(*exc_details)
|
||
|
|
else:
|
||
|
|
cb_suppress = await cb(*exc_details)
|
||
|
|
|
||
|
|
if cb_suppress:
|
||
|
|
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
|
||
|
|
|
||
|
|
|
||
|
|
class nullcontext(AbstractContextManager):
|
||
|
|
"""Context manager that does no additional processing.
|
||
|
|
|
||
|
|
Used as a stand-in for a normal context manager, when a particular
|
||
|
|
block of code is only sometimes used with a normal context manager:
|
||
|
|
|
||
|
|
cm = optional_cm if condition else nullcontext()
|
||
|
|
with cm:
|
||
|
|
# Perform operation, using optional_cm if condition is True
|
||
|
|
"""
|
||
|
|
|
||
|
|
def __init__(self, enter_result=None):
|
||
|
|
self.enter_result = enter_result
|
||
|
|
|
||
|
|
def __enter__(self):
|
||
|
|
return self.enter_result
|
||
|
|
|
||
|
|
def __exit__(self, *excinfo):
|
||
|
|
pass
|