I was pointed here after we found some erroneous behavior.

We have a script run by the system python that invokes a specific venv's python3 with a -m flag requesting a module that is installed (only) in that venv. A user complained that this failed, with the venv's python3 claiming the module was not installed (but it was, as proved by manually running it with the same -m flag).

Eventually someone realized that this was because the system python was a python3 installed by homebrew -- somehow this caused the system python (being python3) set __PYVENV_LAUNCHER__, and that made the venv's python3 ignore the venv's site-packages and instead look in the system python's site-packages.

This feels very wrong.

Maybe this is just a clearer description of https://bugs.python.org/issue31363? But that gained no traction while here there is at least some discussion.