Controlling a venv from the python.exe from another venv does not work since 3.7.2 on Windows. This is probably related to the change

    bpo-34977: venv on Windows will now use a python.exe redirector rather than copying the actual binaries from the base environment.

This is obviously related to bpo-35872, but this could be a different bug.

When a Python script in a venv wants to control another venv by running commands like `another-venv\python.exe -m pip` with subprocess, python.exe is not referencing the other venv. It is referencing to the venv the script currently running from. This is probably because os.environ contains a '__PYVENV_LAUNCHER__' which is pointing to the venv from the script.

This can be reproduced with pipx (https://github.com/pipxproject/pipx-app) by running

    pipx install --python "C:\Program Files (x86)\Python37-32\python.exe" --verbose tox

This results in pip installing to venvs\pipx-app and not in venvs\tox. 


I assume a simpler reproduction might be (but I cannot check this anymore as I'm back on 3.7.1 right now):

    C:\Program Files (x86)\Python37-32\python.exe -m venv C:\Users\$USER\.local\pipx\venvs\tox
    c:\users\$USER\.local\pipx\venvs\pipx-app\scripts\python.exe -c "import subprocess; subprocess.run(['C:\Users\$USER\.local\pipx\venvs\tox\Scripts\python.exe', '-m', 'pip', 'install', 'tox'])"


Downstream bugreport in pipx is https://github.com/pipxproject/pipx-app/issues/81.

So with the fix for multiprocessing, we currently rely on __PYVENV_LAUNCHER__ remaining set throughout the process.

However, it may be better to add a "sys.base_executable" property instead and clear the __PYVENV_LAUNCHER__ variable once we've read it. Then we can set it again in multiprocessing and launch the base executable, and otherwise default to launching the redirector.

New changeset a8474d025cab794257d2fd0bea67840779b9351f by Steve Dower in branch 'master':
bpo-35872 and bpo-35873: Clears __PYVENV_LAUNCHER__ variable (GH-11745)
https://github.com/python/cpython/commit/a8474d025cab794257d2fd0bea67840779b9351f

New changeset 44467e8ea4cea390b0718702291b4cfe8ddd67ed by Steve Dower in branch '3.7':
bpo-35872 and bpo-35873: Clears __PYVENV_LAUNCHER__ variable (GH-11745)
https://github.com/python/cpython/commit/44467e8ea4cea390b0718702291b4cfe8ddd67ed