custom type system using __instancecheck__

customtypesystem.py

class FoobarMeta(type):
    def __instancecheck__(self, obj):
        return hasattr(obj, 'foo') and hasattr(obj, 'bar')

class Foobar(metaclass=FoobarMeta):
    pass

class A:
    def foo(self):
        pass

class B:
    def foo(self):
        pass

    def bar(self):
        pass

class C:
    def bar(self):
        pass

if __name__ == '__main__':
    a, b, c = A(), B(), C()
    assert not isinstance(a, Foobar)
    assert isinstance(b, Foobar)
    assert isinstance(c, Foobar)