GeezerVison

Do we need to call this function every time? Wouldn't be better to call it just once and save it into a private global variable, and use that as the default value for all these methods?

That’s a separate issue already presented in the previous implementation. I’m not sure this is the best plac eto discuss the possible implications in e.g. multiprocessing settings (I don’t even have enough expertise to comment on it).

As it currently stands, the function is only called twice (get_paths and get_path) on import time, so the performance difference is pretty close to none anyway.

Agreed, let's keep the change to the minimal required.

Would be a good opportunity to migrate this to f-strings?

Probably, I’ll combine this to other needed changes, if there are any.

Will the above lookup not raise already? Should this be done before calling _get_preferred_schemes?

The main intention is to guard against incorrect implementation, since _get_preferred_schemes() is expected to be modified by redistributors and implementers. Say an implementer makes a typo:

def _get_preferred_schemes():
    return {
        "prefix": "posix_prefix",
        "home": "posix_home",
        "user": "posix_uesr",  # Oops!
    }

And when the value is later used:

scheme = sysconfig.get_preferred_scheme("user")

...  # Much later...

paths = sysconfig.get_paths(scheme)

This check would catch that posix_uesr is not actually a valid scheme and error early, instead of later when the user actually tries to use the (invalid) scheme.

Can we raise a ValueError with a better message then? "Key '{}' returned scheme '{}' which is not valid on this platform" (or something more correct)

I couldn’t think of a better message and implemented yours for now, any ideas are very welcomed. Ideally I want to point users to the implementer/redistributor instead of reporting the error to CPython, but that’s probably not really feasible.

Can we raise a ValueError with a better message then? "Key '{}' returned scheme '{}' which is not valid on this platform" (or something more correct)