Class in which only one instance of each subclass can exist in a set. Not sure if there is a better way to do this, but it works. Although I don't like that overriding `__eg__` breaks other things.

foo.py

class Foo(object):
  
    def __hash__(self):
        return hash('.'.join([self.__class__.__module__, self.__class__.__name__]))
        
    def __eq__(a, b):
        return a.__hash__() == b.__hash__()


class Bar(Foo):
    pass


class Baz(Foo):
    pass


f1 = Foo()
f2 = Foo()
b1 = Bar()
b2 = Bar()
bz = Baz()

assert f1 == f2  # This is not actually True but needed for set to work
assert f1 is not f2  # Must use `is` to compare not `__eq__`.
assert f1 != b1  # As it should be

instances = [f1, f2, b1, b2, bz]
uniques = set(instances)  # => [f1, b1, bz] albeit unordered
assert len(uniques) == 3 # Yay, it works!
#for i in [f1, b1, bz]:
#    assert i in uniques  # Passes, but pointless; see next loop below
#for i in [f2, b2]:
#    assert i not in uniques  # This fails as it uses `__eq__`

for i in uniques:
    assert i is f1 or i is b1 or i is bz  # That'a better
for i in uniques:
    assert i is not f2 and i is not b2  # Much better, but a pain