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

Using isintsance() is usually fine in __eq__() methods. You shouldn't return False immediately if the isinstance() check fails, though -- it is better to return NotImplemented to give other.__eq__() a chance of being executed:

def __eq__(self, other):
    if isinstance(other, Trout):
        return self.x == other.x
    return NotImplemented

This will become particularly important in class hierarchies where more than one class defines __eq__():

class A(object):
    def __init__(self, x):
        self.x = x
    def __eq__(self, other):
        if isinstance(other, A):
            return self.x == other.x
        return NotImplemented
class B(A):
    def __init__(self, x, y):
        A.__init__(self, x)
        self.y = y
    def __eq__(self, other):
        if isinstance(other, B):
            return self.x, self.y == other.x, other.y
        return NotImplemented

If you would return False immediately, as you did in your original code, you would lose symmetry between A(3) == B(3, 4) and B(3, 4) == A(3).