now we really have gotten OT...
But thanks! that was my question!
Alexander covered the Python part of this, so I'll answer the possible
higher-level question: we haven't yet needed a "negative" leap second, and it's considered unlikely (but not impossible) that we ever will. That's because the Earth's rotation is inexorably slowing ,so the mean solar day inexorably lengthens when measured by SI seconds.
Other things can cause the Earth's rotation to speed up temporarily (like some major geological events), but they've only been able to overcome factors acting to slow rotation for brief periods, and never yet got near to overcoming them by a full second.