diff options
| -rw-r--r-- | Lib/random.py | 7 | ||||
| -rw-r--r-- | Lib/test/test_random.py | 33 |
2 files changed, 37 insertions, 3 deletions
diff --git a/Lib/random.py b/Lib/random.py index 6388f299d3..2ad3809022 100644 --- a/Lib/random.py +++ b/Lib/random.py @@ -252,10 +252,11 @@ class Random(_random.Random): return seq[i] def shuffle(self, x, random=None, int=int): - """x, random=random.random -> shuffle list x in place; return None. + """Shuffle list x in place, and return None. - Optional arg random is a 0-argument function returning a random - float in [0.0, 1.0); by default, the standard random.random. + Optional argument random is a 0-argument function returning a + random float in [0.0, 1.0); if it is the default None, the + standard random.random will be used. """ randbelow = self._randbelow diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py index b5931baf4e..a9aec70f75 100644 --- a/Lib/test/test_random.py +++ b/Lib/test/test_random.py @@ -46,6 +46,39 @@ class TestBasicOps(unittest.TestCase): self.assertRaises(TypeError, self.gen.seed, 1, 2, 3, 4) self.assertRaises(TypeError, type(self.gen), []) + def test_shuffle(self): + shuffle = self.gen.shuffle + lst = [] + shuffle(lst) + self.assertEqual(lst, []) + lst = [37] + shuffle(lst) + self.assertEqual(lst, [37]) + seqs = [list(range(n)) for n in range(10)] + shuffled_seqs = [list(range(n)) for n in range(10)] + for shuffled_seq in shuffled_seqs: + shuffle(shuffled_seq) + for (seq, shuffled_seq) in zip(seqs, shuffled_seqs): + self.assertEqual(len(seq), len(shuffled_seq)) + self.assertEqual(set(seq), set(shuffled_seq)) + + # The above tests all would pass if the shuffle was a + # no-op. The following non-deterministic test covers that. It + # asserts that the shuffled sequence of 1000 distinct elements + # must be different from the original one. Although there is + # mathematically a non-zero probability that this could + # actually happen in a genuinely random shuffle, it is + # completely negligible, given that the number of possible + # permutations of 1000 objects is 1000! (factorial of 1000), + # which is considerably larger than the number of atoms in the + # universe... + lst = list(range(1000)) + shuffled_lst = list(range(1000)) + shuffle(shuffled_lst) + self.assertTrue(lst != shuffled_lst) + shuffle(lst) + self.assertTrue(lst != shuffled_lst) + def test_choice(self): choice = self.gen.choice with self.assertRaises(IndexError): |