In Windows, the default user site-packages location is %APPDATA%\Python\PythonXY\site-package. If %APPDATA% doesn't exist, python won't find the user site-packages folder and won't add it to the python path.

When using tox with the sitepackages = true option, even though a normal virtual environment would have the user site-packages in the python path, it seems that because tox doesn't add %APPDATA% to the subprocess environment, python can't find whatever packages are expected to exist in the user site-packages folder when it is being executed in a toxenv.

This is about the user site-packages folder, not the system one.