[Python-Dev] Update on PEP 523 and adding a co_extra field to code objects

[Chris Angelico]

On Sat, Sep 3, 2016 at 7:56 AM, Brett Cannon <brett at python.org> wrote:
> On Fri, 2 Sep 2016 at 13:31 Dino Viehland via Python-Dev
> <python-dev at python.org> wrote:
>>
>> So it looks like both list and tuple are about within 5% of using co_extra
>> directly.  Using a tuple instead of a list is about a wash except for
>> make_v2 where list is 1.4x slower for some reason (which I didn't dig into).
>>
>> I would say that using a tuple and copying the tuple on updates makes
>> sense as we don't expect these to change very often and we don't expect
>> collisions to happen very often.
>
>
> So would making co_extra a PyTupleObject instead of PyObject alleviate
> people's worry of a collision problem? You're going to have to hold the GIL
> anyway to interact with the tuple so there won't be any race condition in
> replacing the tuple when it's grown (or initially set).
>

I'm not following how this solves the collision problem. If you have a
tuple, how do the two (or more) users of it know which index they're
using? They'd need to keep track separately for each object, or else
inefficiently search the tuple for an object of appropriate type every
time. What am I missing here?

ChrisA