On 12 October 2016 at 11:16, Steven D'Aprano <st...@pearwood.info> wrote: > On Wed, Oct 12, 2016 at 12:25:16AM +0000, Elliot Gorokhovsky wrote: > >> Regarding generalization: the general technique for special-casing is you >> just substitute all type checks with 1 or 0 by applying the type assumption >> you're making. That's the only way to guarantee it's safe and compliant. > > I'm confused -- I don't understand how *removing* type checks can > possible guarantee the code is safe and compliant. > > It's all very well and good when you are running tests that meet your > type assumption, but what happens if they don't? If I sort a list made > up of (say) mixed int and float (possibly including subclasses), does > your "all type checks are 1 or 0" sort segfault? If not, why not? > Where's the safety coming from?
My understanding is that the code does a per-check that all the elements of the list are the same type (float, for example). This is a relatively quick test (O(n) pointer comparisons). If everything *is* a float, then an optimised comparison routine that skips all the type checks and goes straight to a float comparison (single machine op). Because there are more than O(n) comparisons done in a typical sort, this is a win. And because the type checks needed in rich comparison are much more expensive than a pointer check, it's a *big* win. What I'm *not* quite clear on is why Python 3's change to reject comparisons between unrelated types makes this optimisation possible. Surely you have to check either way? It's not that it's a particularly important question - if the optimisation works, it's not that big a deal what triggered the insight. It's just that I'm not sure if there's some other point that I've not properly understood. Paul _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
