PEP 489 is clear that extension modules implementing multi-phase init are expected to support use in multiple interpreters. However, there are two situations where that mandate isn't sufficient:

1. PEP 684 introduces a per-interpreter GIL, where there is an additional thread-safety constraint beyond support for multiple interpreters
2. the HPy project implies modules with multi-phase init but not necessarily supporting multiple interpreters

In both cases a new module def slot (the same one, in fact) is a correct solution.

For per-interpreter GIL, PEP 684 specifies that we must add a module def slot for opting in to supporting per-interpreter GIL.

For HPy, the situation was pointed out to me by @hodgestar during a conversation at PyCon.

CC @encukou

I propose the following solution:

1. add a new Py_mod_multiple_interpreters module def slot
2. interpret the value to indicate support for multiple interpreters and for per-interpreter GIL
3. call _PyImport_CheckSubinterpIncompatibleExtensionAllowed() when appropriate in PyModule_FromDefAndSpec2()

The slot value may be one of the following:

• 0 - does not support multiple interpreters (for HPy)
• 1 - supports multiple interpreters (the default)
• 2 - supports per-interpreter GIL

It would probably make sense to define a constant (macro) for each of those.

Linked PRs

• gh-104108: Add the Py_mod_multiple_interpreters Module Def Slot #104148