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Zope has several workarounds, so isn't affected.

But I agree it could be fixed.  The test is there because it wants to enforce a concept: 'self' should be an instance 
of the class that defines the method (where a subclass instance is fine too).

But the implementation of the test is based too much on the default implementation of classes.

Could someone please submit a patch that replaces the concrete isinstance() test with an isinstance() test similar 
to that in bltinmodule.c?  (Maybe we need to add a new C API, PyObject_IsInstance().)

If someone could come up with a check that conve