Python 3 - 从选项选项返回递归值

我在使用递归方法显示我的值时遇到了一些困难 . 我想输入10个数字，根据选择，将返回最大数字，最小数字和数字之和 . 我得到两个错误：一个是（len）不能返回一个int，如果我在单个函数中输入一个值，它将继续循环，直到程序达到它的解析限制（999） . 我不知道如何继续并获得我想要的值，这似乎是我投入的 Value 观的主要问题 . 任何建议将不胜感激！

def displayMenu():
    print('Enter 1 to find the largest number: ')
    print('Enter 2 to find the smallest number: ')
    print('Enter 3 to sum the list of numbers: ')
    print('Enter 4 to exit: ')

choice = int(input('Please enter your choice: '))

while choice <= 0 or choice >= 5:
    print('Enter 1 to find the largest number: ')
    print('Enter 2 to find the smallest number: ')
    print('Enter 3 to sum the list of numbers: ')
    print('Enter 4 to exit: ')
    choice = int(input('Please enter your choice: '))
else:
    return choice

def main():
    MIN = 1
    MAX = 100
    num_MAX = 10
    user_num = []
    number_list = []
    for i in range(num_MAX):
        user_num = int(input('Please enter a number at this time: '))
    while user_num < 1 or user_num > 100:
        user_num = int(input('Please re-enter numbers between 1 and 100:' ))
        number_list.append(user_num)
    choice = displayMenu()
    while choice != 4:
        if choice == 1:
            largest = find_largest(user_num)
            print('The largest number is: ', largest)
        elif choice == 2:
            smallest = find_smallest(user_num)
            print('The smallest number is: ', smallest)
        elif choice == 3:
            summy = find_sum(user_num)
            print('The sum of the numbers entered is: ', summy)

        choice = displayMenu()

def find_largest(user_num):
    n = len(user_num)
    if n == 1:
        return user_num[0]
    else:
        temp = find_largest(user_num[0:n - 1])
    if user_num[n - 1] > temp:
        return user_num[n - 1]
    else:
        return temp
def find_smallest(user_num):
    n = len(user_num)
    if n == 1:
        return user_num [0]
    else:
        temp = find_smallest(user_num[0:n - 1])
    if user_num[n - 1] < temp:
        return user_num[n - 1]
    else:
        return temp
def find_sum(user_num):
    user_num = [1,2,3,4]
    n = len(user_num)
    if len(user_num) == 1:
        return user_num [0]
    else:
        return user_num[n - 1] + sum[0:n - 1]

main()