class Class_test():
    def fun_1(self):
        self.fun_2()
    def fun_2(self):
        print(1)
Instance = Class_test()
Instance.fun_1()

正确

必须 self才可以

报错

https://blog.51cto.com/u_16055028/8504032

正确

class Class_test():
    def __init__(self):
        pass
    def fun_1(self):
        self.fun_2()
    def fun_2(self):
        print(1)
Class_test().fun_1()