When the setup.py file is in a subdir of the project, the python plugin can't find it and thus finishes without installing anything. To reproduce, let's modify the python-hello test snap: $ mkdir tmp $ cd tmp $ git clone https://github.com/snapcore/snapcraft $ cp -R snapcraft/snapcraft/tests/integration/snaps/python-hello/ . $ cd python-hello $ mkdir subdir $ mv hello subdir/ $ mv setup.py subdir/ $ echo " source-subdir: subdir" >> snap/snapcraft.yaml $ snapcraft