How to generate permutations of a list without “reverse duplicates” in Python using generators

Question

This is related to question How to generate all permutations of a list in Python

How to generate all permutations that match following criteria: if

Answer 1

This is a more concise and faster version of ChristopheD's accepted answer, which I liked a lot. Recursion is great. I made it enforce uniquenss of the incoming list by removing duplicates, however maybe it should just raise an exception instead.

def fac(x): 
    return (1 if x==0 else x * fac(x-1))

def permz(plist):
    plist = sorted(set(plist))
    plen = len(plist)
    limit = fac(plen) / 2
    counter = 0
    if plen==1:
        yield plist
    else:
        for perm in permz(plist[1:]):
            for i in xrange(plen):
                if counter == limit:
                     raise StopIteration
                counter += 1
                yield perm[:i] + plist[0:1] + perm[i:]

# ---- testing ----
plists = [
    list('love'),
    range(5),
    [1,4,2,3,9],
    ['a',2,2.1],
    range(8)]               

for plist in plists:
    perms = list(permz(plist))
    print plist, True in [(list(reversed(i)) in foo) for i in perms]