global 的用法：
num1 = 0
num2 = 0
def change_num():
global num1
num1 = 1
num2 = 1
change_num()
print(num1, num2)
# output: 1, 0
# 不允许直接修改全局变量(数组、字典除外)，需要添加 global 关键字
nonlocal 的用法：
num1 = 0
num2 = 0
def change_num():
num3 = 1
num4 = 1
def change_num_inner():
nonlocal num3
num3 = 2
num4 = 2
change_num_inner()
return num3, num4
num1, num2 = change_num()
print(num1, num2)
# output: 2, 1