direct_assignment_method_replacement.py

class MyClass(object):
    def __init__(self, msg):
        self.msg = msg

    def say_stuff(self):
        print("Standard say stuff message: {}\n\n".format(self.msg))

def say_other_stuff(self):
    print("New say_stuff says this: {}\n\n".format(self.msg))

MyClass.say_stuff = say_other_stuff

ob = MyClass("This is my message here")
ob.say_stuff()