If duck-typing in Python, should you test isinstance?

Question

You have a Python class which needs an equals test. Python should use duck-typing but is it (better/more accurate) to include or exclude an isinstance test in the eq

Answer 1

The "duck-typing" principle is that you don't care what other is, as long as it has a value attribute. So unless your attributes share names with conflicting semantics, I'd suggest doing it like this:

def __eq__(self, other):
    try:
        return self.value == other.value
    except AttributeError:
        return False # or whatever

(Alternately you could test whether other has a value attribute, but "it's easier to ask forgiveness than to get permission")