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import functools
import weakref
from .__wrapt__ import ObjectProxy, _FunctionWrapperBase
# A weak function proxy. This will work on instance methods, class
# methods, static methods and regular functions. Special treatment is
# needed for the method types because the bound method is effectively a
# transient object and applying a weak reference to one will immediately
# result in it being destroyed and the weakref callback called. The weak
# reference is therefore applied to the instance the method is bound to
# and the original function. The function is then rebound at the point
# of a call via the weak function proxy.
def _weak_function_proxy_callback(ref, proxy, callback):
if proxy._self_expired:
return
proxy._self_expired = True
# This could raise an exception. We let it propagate back and let
# the weakref.proxy() deal with it, at which point it generally
# prints out a short error message direct to stderr and keeps going.
if callback is not None:
callback(proxy)
class WeakFunctionProxy(ObjectProxy):
__slots__ = ('_self_expired', '_self_instance')
def __init__(self, wrapped, callback=None):
# We need to determine if the wrapped function is actually a
# bound method. In the case of a bound method, we need to keep a
# reference to the original unbound function and the instance.
# This is necessary because if we hold a reference to the bound
# function, it will be the only reference and given it is a
# temporary object, it will almost immediately expire and
# the weakref callback triggered. So what is done is that we
# hold a reference to the instance and unbound function and
# when called bind the function to the instance once again and
# then call it. Note that we avoid using a nested function for
# the callback here so as not to cause any odd reference cycles.
_callback = callback and functools.partial(
_weak_function_proxy_callback, proxy=self,
callback=callback)
self._self_expired = False
if isinstance(wrapped, _FunctionWrapperBase):
self._self_instance = weakref.ref(wrapped._self_instance,
_callback)
if wrapped._self_parent is not None:
super(WeakFunctionProxy, self).__init__(
weakref.proxy(wrapped._self_parent, _callback))
else:
super(WeakFunctionProxy, self).__init__(
weakref.proxy(wrapped, _callback))
return
try:
self._self_instance = weakref.ref(wrapped.__self__, _callback)
super(WeakFunctionProxy, self).__init__(
weakref.proxy(wrapped.__func__, _callback))
except AttributeError:
self._self_instance = None
super(WeakFunctionProxy, self).__init__(
weakref.proxy(wrapped, _callback))
def __call__(*args, **kwargs):
def _unpack_self(self, *args):
return self, args
self, args = _unpack_self(*args)
# We perform a boolean check here on the instance and wrapped
# function as that will trigger the reference error prior to
# calling if the reference had expired.
instance = self._self_instance and self._self_instance()
function = self.__wrapped__ and self.__wrapped__
# If the wrapped function was originally a bound function, for
# which we retained a reference to the instance and the unbound
# function we need to rebind the function and then call it. If
# not just called the wrapped function.
if instance is None:
return self.__wrapped__(*args, **kwargs)
return function.__get__(instance, type(instance))(*args, **kwargs)
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