Python + Gtk+3 Snap


I have a Debian .deb file which runs my Python/Gtk+3 application.I have also successfully created a python wheel and a pip installation archive. I would like to create a Snap. After dozens of failed attempts, I have copied the code below from another thread with a similar title and commented out some bits I think are irrelevant, to run my application from a desktop file. The process succeeds, but doesn’t create a .snap file. What have I missed out?

version: '0.0.1'
summary: Test
description: Test
confinement: strict
    command: desktop-launch $SNAP/g.desktop
    plugs: [home]

        plugin: python
        source: .
            - python3-distutils-extra
            - python3-gi
            #- gir1.2-unity-5.0
            #- gir1.2-notify-0.7
            - gir1.2-gtk-3.0
            #- gir1.2-pango-1.0
        after: [desktop-gtk3]
#        stage:
#            - -usr/share/pkgconfig # also in desktop-gtk3
#            - -usr/share/mime      # also in desktop-gtk3