That's for per-user isolation. This is about per-project isolation. A single user can have many projects with different dependencies.
that's what PYTHONUSERBASE (the environment variable) is there for!
Ah, I hadn't thought of using it that way. Seems like a working solution, even though it's not the problem that the PEP is meant to solve.
But you may want to keep 3 installation levels: global, per-user and per-project. If you use the second to achieve the third, then you no longer have the second.
PYTHONUSERBASE seems like a superior idea than any others to me. Wonder why it is not popular.