Pythonで階乗を見つける方法

a = int(input('please inputer a integer:'))
num = 1
if a < 0:
    print(&#39;负数没有阶乘！&#39;)
elif a == 0:
    print(&#39;0的阶乘为1！&#39;)
else :
    for i in range(1,a + 1):
        num *= i
    print(num)

#从functools中调用reduce()函数
from functools import reduce
#使用lambda，匿名函数，迭代
num = reduce(lambda x,y:x*y,range(1,7))
print(num)

import math
value = math.factorial(6)
print(value)

def num(n):
    if n == 0:
        return 1
    else:
        return n * num(n - 1)
print(num(6)