理想

a = 0


def hello():
    a += 1
    print(a)


hello()

现实

UnboundLocalError: local variable 'a' referenced before assignment

解决

声明全局变量global

global a
a = 0


def hello():
    global a
    a += 1
    print(a)


hello()

不出意外的话不会报错了，如有意外请拨打114。

参考：

https://www.cnblogs.com/komean/p/11209780.html

https://www.jianshu.com/p/ba7ded1ecbe4