g_num1 = 1 #定义全局变量
g_num2 = 2 #定义全局变量
def add_num():
    global g_num1 #引用全局变量
    g_num1 = 3 #修改全局变量的值
    result = g_num1
    print("result : %d"%result)
def sub_num():
    global g_num2
    g_num2 = 5
    result = g_num2 - 3
    print("result : %d"%result)
add_num()     #输出add_num的值
sub_num()
print("g_num1:%d "%g_num1)
print("g_num2:%d "%g_num2)
#result : 4 result为局部变量
#result : 2
#g_num1:3 全局变量g_num1在执行add_num()函数时被改变
#g_num2:5 全局变量g_num2在执行sub_num()函数时被改变