py3: replace None with -1 to sort an integer array In Python 2: >>> ls = [4, 2, None] >>> sorted(ls) [None, 2, 4] In Python 3: >>> ls = [4, 2, None] >>> sorted(ls) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unorderable types: NoneType() < int() Therefore we replaced None with -1, and the safe part is that, None and -1 are only the keys which are used for sorting so we don't need to convert the -1's back to None.