Determine whether a method was overridden

[porton]

Is the following a good idea, to determine whether a method was overridden with something "real" (and so is not more an "abstract" method which should not be called)?

class Base(object):
    def f(self):
        raise NotImplementedError()

    def hasF(self):
        return self.f != Base.f

class Derived(Base):
    def f(self):
        pass

Or should I instead explicitly define hasF()?

class Base(object):
    def f(self):
        raise NotImplementedError()

    def hasF(self):
        return False

class Derived(Base):
    def f(self):
        pass

    def hasF(self):
        return True

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Hm, I've realized that self.f != Base.f does not do what it should. How to check if a method was overridden?