Factorial of a number in python

Question

def fact(n):
    if n<0:
        return 0
    if (n==0 or n==1):
        return 1
    else:
        n * fact(n-1)

n = int(input("Enter a number"))
fact(n)

i dont know why its giving me a TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'

score 0 · Answer 1 · answered Sep 16 '20 at 20:50

You failed to return a value from your recursion case:

    else:
        n * fact(n-1)

You need

    else:
        return n * fact(n-1)

The default return value is None, so you end up trying to multiply None by n at every level above the second from the bottom.

For instance, when you try fact(3), you recur ...

3 * fact(2)
  2 * fact(1)
    fact(1) is 1 ... return that ...
  2 * 1 is 2 ... but you then return None ...
3 * None ... is illegal

score 0 · Answer 2 · answered Sep 16 '20 at 20:50

0

I believe you meant

return n * fact(n-1)

answered Sep 16 '20 at 20:50

Silvio Mayolo

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Why answer _and_ mark as duplicate? – Pranav Hosangadi Sep 16 '20 at 20:52
@PranavHosangadi My closure was for the "simple typo" reason. A gold Python member came along and had a different idea, which automatically overrides me. – Silvio Mayolo Sep 16 '20 at 20:54
Ah, I see. Questions as basic as this almost always have duplicates though -- better to find one and link it instead of answering. – Pranav Hosangadi Sep 16 '20 at 20:56

Factorial of a number in python

2 Answers2