Christopher Gurnee added the comment: I shouldn't have called this a rounding error issue, that's not really what it is.
A smaller example might help. If I'm given a random int, x, in the range [0, 12), and asked to produce from it a random int, y, in the range (0,8], I've got (at least?) two choices: 1. y = x If x < 8 Else fail 2. y = f(x), where f maps values from [0, 12) -> (0,8] The problem with method 2 is you end up with a mapping like this: 0,1 -> 0 2 -> 1 3,4 -> 2 5 -> 3 6,7 -> 4 8 -> 5 9,10 -> 6 11 -> 7 _randbelow() uses method 1 above. _int(self.random() * istart) is more like method 2. I chose 2^53 * 2/3 just because the bias was easy to demonstrate. There will always be some bias when stop-start % 2^53 != 0, but it might not manifest itself as easily as checking for evenness. Personally, I think 2^52 is still way too high as a cutoff point for using the (presumably faster, I didn't timeit) method 2, but I don't claim to be an expert here.... ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue23974> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
