On Fri, May 28, 2021 at 12:13 PM Brendan Barnwell <brenb...@brenbarn.net> wrote: > > On 2021-05-27 13:15, Chris Angelico wrote: > > Hmmmmmmmm.... let's see. > > > >>>> def merge_shortest(things): > > ... len=len > > ... ... > > ... > >>>> merge_shortest([]) > > Traceback (most recent call last): > > File "<stdin>", line 1, in <module> > > File "<stdin>", line 2, in merge_shortest > > UnboundLocalError: local variable 'len' referenced before assignment > > Okay, yeah, mea culpa. As several people pointed out that doesn't > work. But `len_ = len` does work. However, that doesn't change the > calculus at all for me. My point wasn't about using the exact same > variable name. It's that ANY ability to create a local variable that is > a fast-lookup shortcut for a global one is enough. My point is that > manually creating fast-lookup local-variable shortcuts is inherently a > performance hack and there's no real use in making it slightly > nicer-looking. >
The change of variable name is significant, though. It means that this is no longer a minor change to the function's header; in order to use this optimization, you have to replace every use of the name "len" with "len_" (or "_len"). That isn't necessarily a deal-breaker; I've seen code that optimizes method lookups away by retaining the callable (eg "ap = some_list.append"), so there's uses for that kind of rename; but refactoring becomes harder if you have to be aware of whether you're using the optimized version or not. Why should a performance-improving hoist look ugly? It's a perfectly normal thing to do - calculate something once and reuse the value, because you know that it won't change (or don't care if it changes). It's not a "hack" - it's a legit method of saving effort. (Some day I'll learn how to do this in real life. Why can't I buy just one egg, and then reuse the same egg for every meal?) ChrisA _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/IF5WGNJNQCKYCYMOC7E4NTVJF5WCHJVV/ Code of Conduct: http://python.org/psf/codeofconduct/
