Raise ReferenceError when bound method instance is garbage collected

[bysiber]

WeakFunctionProxy.__call__ checks self._self_instance (a weakref.ref) for truthiness to detect expired references, but weakref.ref objects are always truthy — even after the referent has been garbage collected. Only calling the ref and checking the return value can detect expiration.

When the instance is GC'd but the function still lives in the class dict (the typical case), the code falls through to the unbound-function path and calls the function without self:

class MyClass:
    def method(self):
        return "hello"

obj = MyClass()
proxy = WeakFunctionProxy(obj.method)
del obj  # instance GC'd, but MyClass.method still lives

proxy()
# Expected: ReferenceError
# Actual: TypeError: method() missing 1 required positional argument: 'self'

The fix adds an explicit check: if _self_instance was set (it was a bound method) but the dereferenced value is None, raise ReferenceError directly.

[GrahamDumpleton]

Thanks for the PR.

[GrahamDumpleton]

I have also added some tests specifically for this now as well.

[GrahamDumpleton]

Have released wrapt 2.1.2rc1 on PyPi if want to try with that.