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.
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