when calling a bound method bm of o associated with f, it turns out to call bm.im_func(bm.im_self) where bm.im_func is f and bm.im_self is o.
import types
class A(object):
def __call__(x, y, z):
print('__call__ in A')
print(x, y, z)
class B(object):
pass
a, b = A(), B()
# given two instance a and b where a must have a __call__ bound method
# MethodType creates a bound method c with b as im_self,
# c's underlying function im_func is the __call__ bound method of instane a
c = types.MethodType(a, b)
assert a is c.im_func.__call__.im_self
assert b is c.im_self
assert c.im_func.__call__.im_func is A.__call__.im_func
c(1) # bound method call
c.im_func(c.im_self, 1) # bound method call expansion, c.im_func is an instance with __call__ bound method
c.im_func.__call__(c.im_self, 1) # the __call__ bound method expansion
c.im_func.__call__.im_func(c.im_func.__call__.im_self, c.im_self, 1)
