Avoiding argument checking in recursive calls

[Jervis Whitley]

> I've done this:
>
> def _fact(n):
>    if n = 0: return 1
>    return _fact(n-1)*n
>
> def fact(n):
>    if n < 0: raise ValueError
>    return _fact(n)
>
> but that's ugly. What else can I do?
>
>
Hello, an idea is optional keyword arguments.

def fact(n, check=False):
  if not check:
    if n < 0: raise ValueError
  if n == 0: return 1
  return fact(n - 1, check=True) * n

essentially hiding an expensive check with a cheap one. It saves you
duplicating code in a separate function like in your example.