python变量如何在作用域使用

>>> def func():

    print(a)  # 这种写法是不好的

>>> a = 2     # a是全局作用域变量，但可以被局部作用域读取

>>> func()

2

>>> b = 5

>>> def func(a):

    print(a)

    print(b)    # 尝试打印b变量出错，程序终止

    b = 8

>>> func(3)

3

Traceback (most recent call last):

  File "<pyshell#6>", line 1, in <module>

    func(3)

  File "<pyshell#5>", line 3, in func

    print(b)

UnboundLocalError: local variable 'b' referenced before assignment