Combinations and recursion, from an ASPN snippet

[dromedary]

A short Python program for combinations on ASPN contains this code (I
snipped out the argv[] stuff):

import sys

def printList(alist):
   print ''.join(alist)

def printUniqueCombinations(alist, numb, blist=[]):
   if not numb: 
      return printList(blist)
   for i in range(len(alist)):
      blist.append(alist[i])
      printUniqueCombinations(alist[i+1:], numb-1, blist)
      blist.pop()

def printCombinations(alist, numb, blist=[]):
   if not numb: 
      return printList(blist)
   for i in range(len(alist)):
      blist.append(alist.pop(i))
      printCombinations(alist, numb-1, blist)
      alist.insert(i, blist.pop())

if __name__ == '__main__':
   k='love'
   n=2
   print 'combinations of %d letters in "%s" ' % (n, k)
   printCombinations(list(k), n)
   print 'unique combinations of %d letters in "%s" ' % (n, k)
   printUniqueCombinations(list(k), n)


I'm still rather green with recursion. I understand it in principle,
but I'm not quite following what happens, say, here:

      printUniqueCombinations(alist[i+1:], numb-1, blist)
      blist.pop()

For example, when numb gets to 1, the 

   if not numb:

kicks in. But the recursive loop is still running (on its second
iteration?), so to me it looks as it the next printCombinations() call
gets fed a 0 for numb, making the numb -1 a negative 1. Is that what
finally breaks that loop and allows the next line, the blist.pop(), to
run? Wouldn't the function just accept a negative number?

Thanks for your help. I'd like to see if I can link the combination
code to an anagram maker.

(BTW, is the printCombinations() function above for combinations or
permutations? I'd have thought it was the latter.)

Jon