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

Brett Cannon brett at python.org
Fri Sep 2 18:45:17 EDT 2016

On Fri, 2 Sep 2016 at 15:11 Chris Angelico <rosuav at gmail.com> wrote:

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 coextra >> directly. Using a tuple instead of a list is about a wash except for >> makev2 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 coextra 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?

You're not missing anything, you just have to pay for the search cost, otherwise we're back to square one here of not worrying about the case of multiple users. I don't see how you can have multiple users of a single struct field and yet not have to do some search of some data structure to find the relevant object you care about. We've tried maps and dicts and they were too slow, and we proposed not worrying about multiple users but people didn't like the idea of either not caring or relying on some implicit practice that evolved around the co_extra field. Using a tuple seems to be the best option we can come up with short of developing a linked list which isn't that much better than a tuple if you're simply storing PyObjects. So either we're sticking with the lack of coordination as outlined in the PEP because you don't imagine people using a combination of Pyjion, vmprof, and/or some debugger simultaneously, or you do and we have to just eat the performance degradation. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20160902/692160cb/attachment-0001.html>

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