Python: testing

I am impressed! When writing the header of a function, why not simply write an example and automatically test the function?

#!/usr/bin/env python

import doctest

def fact(n, stop=1, r=1):
 """ An iterative factorial function.
 n!     = n*(n-1)*(n-2)*..*(n-(n-stop-1))*r
 stop    fact(5,3)= 5*4
 r        fact(5,3,10)=5*4*10
 >>> fact(5,3,10)
 200
 >>> fact(4)
 24
 """

 while n > stop :
   r *= n
   n -= 1
   return r    

def main():
 doctest.testmod()
 return 0

if __name__ == '__main__': main()

More here: http://docs.python.org/tutorial/stdlib.html#quality-control