[Python-Dev] Intricacies of calling eq (original) (raw)

Maciej Fijalkowski fijall at gmail.com
Wed Mar 19 14:09:04 CET 2014

On Wed, Mar 19, 2014 at 2:42 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Tue, 18 Mar 2014 09:52:05 +0200 Maciej Fijalkowski <fijall at gmail.com> wrote:

We're thinking about doing an optimization where say: if x in d: return d[x] where d is a dict would result in only one dict lookup (the second one being constant folded away). The question is whether it's ok to do it, despite the fact that it changes the semantics on how many times eq is called on x. I don't think it's ok. If the author of the code had wanted only one lookup, they would have written: try: return d[x] except KeyError: pass I agree that an eq method with side effects is rather bad, of course. What you could do is instruct people that the latter idiom (EAFP) performs better on PyPy. Regards Antoine.

I would like to point out that instructing people does not really work. Besides, other examples like this:

if d[x] >= 3: d[x] += 1 don't really work.

More information about the Python-Dev mailing list