modify class in running

mock_class.py

class Lmy(object):
    def sum(self, a, b, c=None):
        if c is None:
            return a + b
        else:
            return a + b + c

    def do_data(self, data):
        a, b = data
        return self.sum(a, b)


def new_sum(func, c):
    def new_func(a, b):
        return func(a, b, c)

    return new_func


a = Lmy()
old_sum = a.sum
a.sum = new_sum(old_sum, 5)
a.do_data((1, 2))