Calling PyObject_LengthHint() adds an overhead. It is significant for short sequences. I work on a patch that reduces it. PR 6493 adds the second call of PyObject_LengthHint() and increases the overhead. As for this issue, in-place repeat overallocates too. >>> a = [0]; a *= 10; getsizeof(a) 200 I think it would be better to make it not preallocating. And maybe it would be worth to avoid overallocating if newsize > allocated + allocated/8 or something like.