from types import MethodType

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

def new_method(self, value):
    return self.val + value

obj = MyObj(3)
# only works for obj
obj.method = MethodType(new_method, obj, MyObj)

MyObj.method = MethodType(new_method, None, MyObj)