Axiom of Choice?

axiom.py

import random

def choose(s, choice=None):
    if choice is None:
        choice = random.choice

    if len(s) == 0:
        raise KeyError("Cannot choose from empty set")

    return choice(list(s))

def main(s):
    if len(s) == 1:
        print(choose(s))
    else:
        print(sorted(s))

if __name__ == "__main__":
    import sys
    main(frozenset(sys.argv[1:]))   

test_axiom.py

import unittest
from operator import itemgetter

import axiom

class TestAxiom(unittest.TestCase):
    def test_empty(self):
        self.assertRaises(
            KeyError,
            axiom.choose, frozenset())

    def test_errors(self):
        s = frozenset({"a", "b", "c"})
        self.assertRaises(
            IndexError,
            axiom.choose, s, itemgetter(4))

    def test_chooser_pop(self):
        s = frozenset({"a", "b", "c"})
        self.assertEqual(
            axiom.choose(s, list.pop),
            list(s).pop())

    def test_chooser_itemgetter(self):
        s = frozenset({"a", "b", "c"})
        self.assertEqual(
            axiom.choose(s, itemgetter(1)),
            list(s)[1])

    def test_chooser_random(self):
        s = frozenset({"a", "b", "c"})

        self.assertTrue(
            axiom.choose(s) in s)

see https://gist.github.com/3253145