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

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

Start with the PyConfig documentation and the module_search_paths_set behavior described in the issue, then read the linked Discourse discussion for prior design context. Define how application-specific paths should be combined with the default sys.path and what the documented API behavior should be; completion should include an agreed implementation and corresponding documentation or tests.