[Python-Dev] Can we split PEP 489 (extension module init) ?

[Stefan Behnel]

Hi,

coming back to PEP 489 [1], the multi-phase extension module
initialization. We originally designed it as an "all or nothing" feature,
but as it turns out, the "all" part is so difficult to achieve that most
potential users end up with "nothing". So, my question is: could we split
it up so that projects can get at least the main advantages: module spec
and unicode module naming.

PEP 489 is a great protocol in the sense that it allows extension modules
to set themselves up in the same way that Python modules do: load, create
module, execute module code. Without it, creating the module and executing
its code are a single step that is outside of the control of CPython, which
prevents the module from knowing its metadata and CPython from knowing
up-front what the module will actually be.

Now, the problem with PEP 489 is that it requires support for reloading and
subinterpreters at the same time [2]. For this, extension modules must
essentially be free of static global state, which comprises both the module
code itself and any external native libraries that it uses. That is
somewhere between difficult and impossible to achieve. PEP 573 [3] explains
some of the reasons, and lists solutions for some of the issues, but cannot
solve the general problem that some extension modules simply cannot get rid
of their global state, and are therefore inherently incompatible with
reloading and subinterpreters.

I would like the requirement in [2] to be lifted in PEP 489, to make the
main features of the PEP generally available to all extension modules.

The question is then how to opt out of the subinterpreter support. The PEP
explicitly does not allow backporting new init slot functions/feeatures:

"Unknown slot IDs will cause the import to fail with SystemError."

But at least changing this in Py3.8 should be doable and would be really nice.

What do you think?

Stefan



[1] https://www.python.org/dev/peps/pep-0489/
[2]
https://www.python.org/dev/peps/pep-0489/#subinterpreters-and-interpreter-reloading
[3] https://www.python.org/dev/peps/pep-0573/