On 05/25/2017 09:01 PM, Eric Snow wrote:
On Thu, May 25, 2017 at 11:19 AM, Nathaniel Smith <n...@pobox.com> 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.
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