What a welcome return to the office after visiting the seaside over the weekend! The second day of EnHackathon felt much more organised than the first, which hopefully helped everyone to able to make progress in their area of focus. We were also graced with the presence of a couple of new contributors, bringing our group up to 9 people.

Subinterpreters Continued

I spent the day continuing progress on the two subinterpreter issues I was looking at last time. It was great to have Phil on board looking into the other issues I mentioned last time (check out his blog post), and also great to have Ben helping me out towards the end of the day.

‘Tracing Possible’ Field


This issue suggests copying a tracing_possible field from the CPython runtime state struct to the interpreter state struct.

It seems this field records how many threads have tracing enabled, where tracing is the mechanism used by tools such as PDB and code coverage analysers. If the value is zero, this allows the code execution to follow the ‘fast path’.

It’s still not clear to me what the full scope of the required change is, but I’ve put my initial attempt up as a PR within my cpython repo. I’m also unsure how this ties in with previous attempts to move fields from the runtime ceval struct to a per-interpreter ceval struct… I’m at the point it would be good to get some more input from a maintainer of the project.

New ‘List Channel Interpreters’ API


As far as I can tell, this issue tracks the addition of an API on the internal _interpreters module (Modules/_xxsubinterpretersmodule.c) to list interpreters associated with [an end of?] a channel.

Looking at the section detailing the API for the new interpreters module in PEP 554, it seems that the RecvChannel and SendChannel should have an interpreters property, which should return Interpreter objects (as opposed to interpreter IDs). The logic of associating channels with interpreters seems to have already been implemented in the internal module, so this task should just be a matter of extracting the association. However, the Interpreter type is defined in the external module, so as far as I can tell it makes most sense for the internal module to return interpreter IDs, and for the external module that wraps the internal module to convert the IDs into Interpreter objects.

I’ve managed to more-or-less implement a proof-of-concept for the internal channel_list_interpreters() API (thanks to Ben for his help fixing my broken C code), with the main thing left to sort out being how to handle the distinction between send and receive channels (maybe just a boolean argument?). I’ve created a PR in my cpython repo to track my progress with this and hopefully make it easier for any maintainers to give me feedback.


I’m definitely at a point where some input from a maintainer would be good. Phil managed to complete the open issue about forking, which leaves us with one open issue that I was recommended to take a look at by Eric. There is now interest in the subinterpreters project from 4 of us taking part in EnHackathon, so it would be good to work out what we should look into ahead of the next day of contribution.

I’ve continued to find the subinterpreter project interesting to work on, as it involves very general CPython skills/knowledge such as: getting familiar with the CPython runtime, implementing a Python module in C and interfacing with a pure Python module, and debugging CPython code at the C level. I’m hoping that as I increase my knowledge around this project it will become easier to understand the big picture more, and therefore easier to dive in with the changes that interest me the most.

Some more references (docs for the C API) that I found useful this time were:

Overall Thoughts

In the EnHackathon environment I find it much easier working with others than working alone, although in general I find I also work well in an isolated environment. I feel like the choice to work on larger pieces of work in small groups was a good one, and is something we could still aim to do more. Personally I see this time as a kick-starter for working on CPython in my own time - it would be great if other people taking part in EnHackathon would be interested in doing the same.

Although I received a reasonable amount of support from the Python community when I emailed before EnHackathon started, it’s been hard to translate that into feeling there are people we can go to with the types of things that people get stuck on. It seems like a lot of the time what people need is the ability to put together a draft patch and get some feedback quickly, to allow for iterations to be made without losing interest or deciding they might as well keep trying without input from an experienced developer. It seems like the best way we currently have to do that is to attach a patch to an issue, and maybe add someone to the nosy list (Kyle Stanley was kind enough to suggest we could add him to nosy lists or ‘@’ mention on GitHub for issues we’re working on). Perhaps if other devs would be happy to mentor in this way it would [artificially] feel like the level of support was higher? It feels to me like having a discussion on a PR open in our personal cpython repos would be preferable so that we (as new contributors) don’t have to worry about creating a messy history in a very public record of an issue such as on BPO.

What’s Next?

We’re planning on having a couple of days in a row doing EnHackathon next week to get some momentum going - probably Weds-Thurs 20th-21st November. It looks like we’ll be continuing with about 10 people. With everyone gradually getting more familiar with the code and the process I’m looking forward to continuing our successful contributions!