Embedded interpreters: No way to extend `sys.path`

[smurfix]

Feature or enhancement

Proposal:

Setting up an embedded Python interpreter often involves augmenting sys.path with some application-specific subdirectories, so as not to pollute the system's normal Python installation.

Unfortunately, PyConfig only affords either replacing the default path wholesale, or leaving it as-is (via module_search_paths_set). There appears to be no way to augment sys.path with application-specific directories and no documented way to retrieve the default contents of sys.path.

A workable solution might be to enhance cfg.module_search_paths_set so that when it's set to 2 or 3, the application's paths shall be prepended / appended to sys.path instead of replacing it.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/embedding-how-do-i-extend-sys-path/75734

[ZeroIntensity]

This looks like a duplicate of gh-126977, but I think we can leave that one open for tracking and use this for the actual implementation. But we should discuss what the best way to do this is (e.g. should we just have a PyConfig field for sys.path, or something more broad?)