Why not write it yourself? It's a one-liner or such:
from operator import mul # or mul=lambda x,y:x*y
from fractions import Fraction
def nCk(n,k):
return int( reduce(mul, (Fraction(n-i, i+1) for i in range(k)), 1) )
Test - printing Pascal's triangle:
>>> for n in range(17):
... print ' '.join('%5d'%nCk(n,k) for k in range(n+1)).center(100)
...
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 28 56 70 56 28 8 1
1 9 36 84 126 126 84 36 9 1
1 10 45 120 210 252 210 120 45 10 1
1 11 55 165 330 462 462 330 165 55 11 1
1 12 66 220 495 792 924 792 495 220 66 12 1
1 13 78 286 715 1287 1716 1716 1287 715 286 78 13 1
1 14 91 364 1001 2002 3003 3432 3003 2002 1001 364 91 14 1
1 15 105 455 1365 3003 5005 6435 6435 5005 3003 1365 455 105 15 1
1 16 120 560 1820 4368 8008 11440 12870 11440 8008 4368 1820 560 120 16 1
>>>
PS. edited to replace int(round(reduce(mul, (float(n-i)/(i+1) for i in range(k)), 1)))
with int(reduce(mul, (Fraction(n-i, i+1) for i in range(k)), 1))
so it won't err for big N/K