第一种方法：通过排序，切片，取倒数第二个值

list1 = [1,2,6,4,5,7]
list2 = sorted(list1)
list3= list2[-2]
print(list3)

第二种方法：循环列表中的值

list1 = [1,2,6,4,5,8,10,12,13,15]
one = list1[0]
two = list1[1]
for i in range(1,len(list1)):
    if list1[i] > one:
        two = one
        one = list1[i]
    elif list1[i] > two:
        two = list1[i]
    else:
        pass
print(two)