Question

Hi I am overriding __cmp__ . If the second object passed is None, or if it is not an instance of someClass, then it returns -1.

I don’t understand what exactly is happening here.

class someClass():
    def __cmp__(self, obj):
        if obj == None:
            return -1
        if not isinstance(obj, someClass):
            return -1  

My test function:

def test_function(self):  
        obj1 = someClass()
        self.assertTrue(obj1 < None)
        # I get true.
        self.assertTrue(obj1 > None)
        # I get failure as False is returned.

Could anyone please explain to me:

• What are the return values?
• How is it deciding whether it will return True or False when the comparison signs are changed?

Answer 1

When the obj is None, your program will return -1, while returning a negative integer means self < obj, so obj1 < None is True, and obj1 > None is false.