Feature or enhancement

Currently sys.path[0] is set by pymain_run_python() (in Modules/main.c). This happens after pymain_init(), which initializes the runtime, including the rest of sys.path (via getpath.py and site.py). This makes it harder to reason about and introduces extra complexity for subinterpreters. (See gh-109793 and gh-109794.)

We should consider calculating sys.path[0] and setting it to its own PyConfig field via getpath.py, when the rest of the base sys.path is calculated. We may need a later check to verify that there is a matching importer, as pymain_run_python() does. (FWIW, it isn't clear that there's any value to storing the sys.path[0] value on the global _PyPathConfig.)

Also, we currently wait to actually set sys.path[0] (for the main interpreter) until after the readline/rlcompleter modules are imported in pymain_run_python(). We'd need to factor that in.

CC @zooba, @vstinner, @ncoghlan

Linked PRs

• gh-109853: Fix sys.path[0] For Subinterpreters #109994
• [3.12] gh-109853: Fix sys.path[0] For Subinterpreters (gh-109994) #110701