python什么时候加self？

class A():

    def mutl(self):

        x=5

    def sing(self):

        y=5*x

        print(y)

a=A() 

a.mutl()

a.sing()

错误如下：

<ipython-input-3-e305f6d63af0> in sing(self)

      3         x=5

      4     def sing(self):

----> 5         y=5*x

      6         print(y)

      7 #     def print1(self):

NameError: name 'x' is not defined

class A():

    def mutl(self):

        self.x=5

    def sing(self):

        y=5*self.x

        print(y)

a=A() 

a.mutl()

a.sing()

打印结果：

25