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pythonequality

Understanding the details of equality in Python

When trying to construct an example where a == b is not the same as b == a, it seems that I have accidentally constructed an example where a == b is not the same as a.__eq__(b):

class A:

    def __eq__(self, other: object) -> bool:
        return type(other) is A


class B(A):
    pass


if __name__ == '__main__':
    a = A()
    b = B()
    assert not a.__eq__(b)  # as expected
    assert not (a == b)  # Why does this fail?

Can somebody explain to me why the last assertion fails? I expected it to be the same as the second one.

Solution

  • The relevant quote explaining what happens is located in the documentation:

    If the operands are of different types, and the right operand’s type is a direct or indirect subclass of the left operand’s type, the reflected method of the right operand has priority, otherwise the left operand’s method has priority. Virtual subclassing is not considered.