class A(object):
 def __init__(self):
 print "In A"

 def __getattr__(self):
 return 0

 def __add__(self, a):
 return 100

class B(A):
 pass

a = A()
print (a + a)

def foo(self, rhs):
 return 200

b = B()
b.__add__ = foo
print (b + b)