我在Python 2.7.9中的代码有什么问题？

Question

I have just started learning python and written a procedure to calculate factorial of a number. I am getting a logical error. The value returned by fact function is None and the value of factorial after execution is 24.

factorial = 1

def fact(num) :
    if num == 0 :
        return 1
    global factorial
    print factorial
    factorial *= num
    if num-1 > 1 :
        fact(num - 1)
    else :
        return factorial

print fact(4)
print factorial

Output :

1
4
12
None
24

Answer 1

You should change

if num-1 > 1 :
    fact(num - 1)
else:
    return factorial

to:

if num-1 > 1 :
    return fact(num - 1)
else :
    return factorial

The problem was you were not returning anything except for the base case.