Python的全局变量竟然不用先声明

def discounts(price,rate) :
    final_price = price * rate
    print(old_price)
    return final_price

old_price = 120.0
rate = 0.9
new_price = discounts(old_price,rate)
print(new_price)

这个程序竟然没错，明明old_price还没被定义啊