# Concerns About PyUnstable\_Module\_SetGIL()

**URL:** <https://discuss.python.org/t/concerns-about-pyunstable-module-setgil/89374>\
**Category:** Core Development\
**Created:** [April 23, 2025, 6:32pm UTC](https://discuss.python.org/t/concerns-about-pyunstable-module-setgil/89374 "2025-04-23T18:32:45Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![eric.snow](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/eric.snow/32/31_2.png) [@eric.snow](https://discuss.python.org/u/eric.snow)\
**Post date:** [April 23, 2025, 6:34pm UTC](https://discuss.python.org/t/concerns-about-pyunstable-module-setgil/89374/2 "2025-04-23T18:34:20Z")

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As I noted, implementing multi-phase init is fairly trivial \[1\]:

1. move the content of the module’s init function to a corresponding “module exec” function (dropping the call to create the module object)
2. set the module def’s [`m_slots`](https://docs.python.org/3/c-api/module.html#c.PyModuleDef.m_slots) field to an array with:
  - a `Py_mod_exec` slot, set to the new module exec function
  - a `Py_mod_multiple_interpreters` slot \[2\], set to `Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED` \[3\]
  - to try free-threading, a `Py_mod_gil` slot \[4\], set to `Py_MOD_GIL_NOT_USED `

3. set `def.m_size` to 0 (if negative)
4. update the module init function to only `return PyModuleDef_Init(def);` for the corresponding module def

(Also see [Module Objects — Python 3.13.3 documentation](https://docs.python.org/3/c-api/module.html#multi-phase-initialization) and [https://docs.python.org/3/howto/isolating-extensions.html.](https://docs.python.org/3/howto/isolating-extensions.html.))

There’s no need to do any of the isolation stuff (heap types, module state, etc.) immediately.

At that point, the only extra step to run on a free-threading build would be to add the `Py_mod_gil` slot, set to `Py_MOD_GIL_USED `, etc.

IMHO, the free-threading build should always treat single-phase init modules as though they had `Py_MOD_GIL_USED ` set.

* * *

1. my original outline of the steps: [https://github.com/python/cpython/pull/116882#discussion\_r1586449047](https://github.com/python/cpython/pull/116882#discussion_r1586449047) 

2. the default is `Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED` 

3. think of `Py_mod_multiple_interpreters` as a hypothetical `Py_mod_isolated`, where `Py_MOD_PER_INTERPRETER_GIL_SUPPORTED` means “isolated” and `Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED` means “not isolated” 

4. the default is `Py_MOD_GIL_USED`

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_[View the full topic](https://discuss.python.org/t/concerns-about-pyunstable-module-setgil/89374)._