Documentation of sys.base_prefix is unclear when a virtual environment is created from another virtual environment

[pfmoore]

Documentation of the sys module

The documentation of sys.base_prefix states that "base_prefix and base_exec_prefix will remain pointing to the base Python installation (the one which the virtual environment was created from)"

In the case of creating one virtual environment from another:

python -m venv foo --without pip
foo/Scripts/python.exe -m venv bar --without-pip

sys.base_prefix in bar points to the system Python installation, not to foo. (Tested on Windows). I believe this behaviour is better, and I'm suspect that existing tools rely on it, but regardless of what is correct behaviour, the documentation should be more explicit about what the expected value of sys.base_prefix is in a virtual environment created from another virtual environment.

Also, given that the behaviour is documented as being (at least in part) implemented in site.py, the behaviour when the -S flag ("don't import site") is supplied to Python should be documented as well.