python指定下标分别逆序

指定下标分别逆序
import random
a=list([random.randint(1,100) for i in range(1,31)])
def fc3(a:list,b:int):
    if b<0 or b>len(a):
        return "Error"
    return a[:b][::-1]+a[b:][::-1]
print(a,"\n",fc3(a,5))