ambig2.py

class Test(object):
    def __init__(self):
        self.val = None

    def __eq__(self, rhs):
        return self.val == None

t_0 = Test()

print(t_0 == None) # True
print(None == t_0) # True

print(t_0 is None) # False
print(None is t_0) # False

ambiguity.py

class Test(object):
    def __init__(self, val):
        self.val = val 

    def __eq__(self, rhs):
        return self.val == rhs.val

t_0 = Test(0)
t_1 = Test(0)

print(t_0 == t_1)
print(t_0 is t_1)