How to fully understand and implement a recursive factorial function?

Lately I found out that I can't get what I want to have, like getting the Negative Integers of the factorial of this:

def rec_fac(n):
   if n == 1:
       return n
   else:
       return n*rec_fac(n-1)

if you have something to add with this code.. Leave a comment =)

thanks in advance!

Not sure what the question is, but factorial is defined for natural numbers.

So to handle negative numbers, do this:

def rec_fac(n):
   if n < 1:
       raise NotImplementedError('no definition for rec_fact(x) for x < 1')
   if n == 1:
       return n
   else:
       return n*rec_fac(n-1)