4.3 Janus and virtual environments (venv)

An embedded Python system does not automatically pick up Python virtual environments. It is supposed to setup its own environment. Janus is sensitive to Python venv environments. Running under such as environment is assumed if the environment variable VIRTUAL_ENV points at a directory that holds a file pyvenv.cfg. If the virtual environment is detected, the actions in the list below are taken.^{3This is based on observing how Python 3.10 on Linux responds to being used inside a virtual environment. We do not know whether this covers all platforms and versions.}

• Initialize Python using the -I flag to indicate isolation.
• Set sys.prefix to the value of the VIRTUAL_ENV environment variable.
• Remove all directories with base name site-packages or dist-packages from sys.path.^{4Note that -I only removes the personal packages directory, while the Python executable removes all, so we do the same.}
• Add $VIRTUAL_ENV/lib/pythonX.Y/site-packages to sys.path, where X and Y are the major and minor version numbers of the embedded Python library. If this directory does not exist we print a diagnostic warning.
• Add a message to py_version/0 that indicates we are using a virtual environment and from which directory.