I suggest adding proxy support to isinstance along the
lines of  the following pseudocode.

if object is instance of ProxyTypes:
    check using object.__class__ (which is accessed via
the proxy)
else:
    perform normal isinstance checking.

weakref.proxy is intended to hide the details of using
weakrefs . The illusion is dispelled when using
isinstance. Ideally isinstance shouldn't be modified
and the proxy class should just do the right thing, but
I am not sure that is feasable to attain that level of
info hiding while retaining the abitily to do both
isinstance(x, weakref.ProxyTypes) and isinstance(x,
xParentTypes).