Proves you can deploy a Python 3.9 program as a snap, using core20 which only comes with Python 3.8 built in.
Based on the thread https://forum.snapcraft.io/t/build-a-snap-with-any-version-of-python-i-want/10420/8
snapcraft
sudo snap install --devmode --dangerous *.snap
which will build and install the snap. Then to run it - twice with different entry points:
andy-snap-python-hello
andy-snap-python-hello.python-v
The $SNAP reference
command: bin/python3 $SNAP/src/main.py
refers to the prime directory, which is what is ultimately copied into the resulting snap file created. The reference to python is bin/python which could be $SNAP/bin/python3 as the location of the venv/bin directory seems to end up in the prime directory as bin. The command could be simplified to
command: bin/python3 src/main.py
confirmed. That works.
$ unsquashfs -l *.snap|less
where squashfs-root/ is the prime directory during build. E.g.
squashfs-root
squashfs-root/bin
squashfs-root/bin/pip
squashfs-root/bin/pip3
squashfs-root/bin/pip3.9
squashfs-root/bin/python
squashfs-root/bin/python3
squashfs-root/bin/python3.9
squashfs-root/src/main.py
etc.
After installation
$ ls /snap/andy-snap-python-hello/
to see what is actually being copied to your file system from the .snap file - this is what is being ultimately run.
Unfortunately I couldn't get it working with core18 since I got the error
Could not find a required package in 'build-packages': python3.9-venv