> should the function be expanded to calculate for negative 
> n or is the function expected to work only in combination sense?

If this were my design, I would offer both but in separate functions:

def comb(n, k):
    if n < 0: raise ValueError
    return bincoeff(n, k)

def bincoeff(n, k):
    if n < 0:
        return (-1)**k * bincoeff(n+k+1, k)
    else:
        # implementation here...

I believe we agreed earlier that supporting non-integers was not 
necessary.

Are you also providing a perm(n, k) function?