In the first patch the counter was placed in the _dictkeysobject structure. In the second place it is placed in the PyDictObject so it now has no memory cost. Access time to new counter for non-modifying operations is same as in current code. The only additional cost is time cost for modifying operations. But modifying operations is usually much rare than non-modifying operations, and the incrementing one field takes only small part of the time needed for all operation. I don't think this will affect total performance of real programs.