Using recursion in python 3 to get a factorial number

Question

I have this little code in python 3 to get a factorial number:

def factorial(n):
if n == 0:
return 1;
else:
return n * factorial(n - 1);

I don't understand the last line... please can someone explain me what is going on?

Answer 1

I think the explanation here is quite okay:
https://www.sololearn.com/learn/JUMP_LINK__&&__Python__&&__JUMP_LINK/2465/
Basically, for 5 it would go like:
f(5) = 5 * f(5-1)
f(5-1) = (5-1) * f(5-1-1)
f(5-1-1) = (5-1-1) * f(5-1-1-1)
f(5-1-1-1) = (5-1-1-1) * f(5-1-1-1-1)
f(5-1-1-1-1) = (5-1-1-1-1) * f(5-1-1-1-1-1)
f(5-1-1-1-1-1) = f(0), so here we return 1 -- it's the so-called base case.
Now, if you replace all f(x) with relevant input, you get 5*4*3*2*1*1 = 120, which is what f(5) is equal to.

Answer 2

there is a useful link for a youtube video there by the comments, have a look there.

Answer 3

the last line is the actual recursion.
keep calling the function factorial(n), reducing n by 1 in each call until n == 0 then stop and return 1

Answer 4

thanks!