[Python-Dev] Intricacies of calling __eq__

[Maciej Fijalkowski]

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.