[Python-Dev] Intricacies of calling __eq__

[Paul Moore]

On 18 March 2014 19:46, Maciej Fijalkowski <fijall at gmail.com> wrote:
>> A question: how far away will this optimization apply?
>>
>>     if x in d:
>>         do_this()
>>         do_that()
>>         do_something_else()
>>         spam = d[x]
>
> it depends what those functions do. JIT will inline them and if
> they're small, it should work (although a modification of a different
> dict is illegal, since aliasing is not proven), but at some point
> it'll give up (Note that it'll also give up with a call to C releasing
> GIL since some other thread can modify it).

Surely in the presence of threads the optimisation is invalid anyway
as other threads can run in between each opcode (I don't know how
you'd phrase that in a way that wasn't language dependent other than
"everywhere" :-)) so

    if x in d:
        # HERE
        spam = d[x]

d can be modified at HERE. (If d is a local variable, obviously the
chance that another thread has access to d is a lot lower, but do you
really do that level of alias tracking?)

Paul