On 05/25/2017 09:01 PM, Eric Snow wrote:
On Thu, May 25, 2017 at 11:19 AM, Nathaniel Smith firstname.lastname@example.org wrote:
My impression is that the code to support them inside CPython is fine, but they're broken and not very useful in the sense that lots of C extensions don't really support them, so in practice you can't reliably use them to run arbitrary code. Numpy for example definitely has lots of subinterpreter-related bugs, and when they get reported we close them as WONTFIX.
Based on conversations at last year's pycon, my impression is that numpy probably *could* support subinterpreters (i.e. the required apis exist), but none of us really understand the details, it's the kind of problem that requires a careful whole-codebase audit, and a naive approach might make numpy's code slower and more complicated for everyone. (For example, there are lots of places where numpy keeps a little global cache that I guess should instead be per-subinterpreter caches, which would mean adding an extra lookup operation to fast paths.)
Thanks for pointing this out. You've clearly described probably the biggest challenge for folks that try to use subinterpreters. PEP 384 was meant to help with this, but seems to have fallen short. PEP 489 can help identify modules that profess subinterpreter support, as well as facilitating future extension module helpers to deal with global state. However, I agree that *right now* getting extension modules to reliably work with subinterpreters is not easy enough. Furthermore, that won't change unless there is sufficient benefit tied to subinterpreters, as you point out below.
PEP 489 was a first step; the work is not finished. The next step is solving a major reason people are using global state in extension modules: per-module state isn't accessible from all the places it should be, namely in methods of classes. In other words, I don't think Python is ready for big projects like Numpy to start properly supporting subinterpreters.
The work on fixing this has stalled, but it looks like I'll be getting back on track. Discussions about this are on the import-sig list, reach out there if you'd like to help.