I ran into a strange bug while experimenting with metaclasses that implement a custom mro() method. It only seems to occur in interactive mode, either returning completely unrelated values or causing a segfault. Python 2 appears unaffected. I have been able to reproduce this with the following code:

# $ python -i weird.py
# >>> proxy.x
# 52011448
# >>> proxy.x
# 6160
# ...

class Foo:
    pass

class Meta(type):
    def mro(cls):
        return (cls, Foo, object)

    def __setattr__(cls, name, value):
        setattr(Foo, name, value)

proxy = Meta('FooProxy', (), {})

proxy.x = 300
proxy.x  # don't omit
proxy.x = 0