snapcraft 4.0, on a core20 snap Python libraries and apps installed by a python part (via pip or setup.pyt dependencies) are installed in $SNAP/lib/python3.8/site-packages, but the path is not added to sys.path: sys.path = [ '/', '/usr/lib/python38.zip', '/usr/lib/python3.8', '/usr/lib/python3.8/lib-dynload', '/usr/lib/python3/dist-packages', ] As such, those modules are not found when running the app.