Nested virtual environment support in site/venv modules

Feature or enhancement

Proposal:

It's a bit of a niche use case, but it would be helpful if you could have one virtual environment be created that references another as its parent. If you currently attempt to do this, the pyvenv.cfg of the child virtual environment only indirectly references the parent through the command variable (which doesn't appear to be used). The code in site.venv could be enhanced to infer the prefix of the parent environment and add those site packages after the current virtual environment. This would ensure that the order of sys.path entries would be appropriate.

This would allow you to create a group virtual environment with a lot of common packages and still allow users to have their own child environments with additional or newer packages.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Start in Lib/site.py at the site.venv implementation referenced by the issue, and inspect how pyvenv.cfg and the command variable are currently handled. Define how a child environment should infer its parent prefix and order the parent site packages after the child’s entries; done means nested environments expose the intended sys.path ordering.