I just got the notice that python 3.9 is going to lose support soon, I think at the end of Oct '25, and I'd rather not reinstall using a new image.
Currently using Rasperry Pi Zero 2 W, with OctoPI, Debian 11 - bullseye.
and Python 3.9.2
What should this be because this post used 3.11.9
but is that just an arbitrary choice or what is the best one?
Use the latest supported version.
At the time of writing this is Python 3.13.
For the record, Python 3.9 will become EOL by Oct 31st 2025. OctoPrint will continue to support it throughout the lifecyle of the not even release 1.12.x version. OctoPrint 1.13.0, which will drop support for Python 3.9, by current plans won't release until late 2026 or early 2027.
edit
I've also added a table to the FAQ entry about this:
I understand being cautious and avoiding potential troubles for users, but a moderately advanced solution should include the simple:
sudo nano /etc/apt/sources.list
Replacing bullseye with bookworm and save
sudo apt update
sudo apt full-upgrade
This will install Python 3.11, migrate OctoPrint (like any other package) and upgrade the entire operating system.
Granted, a backup is always the most sensible thing to do before starting this procedure but unless you have a pretty messed-up OS this would lead to a clean upgrade in a matter of minutes without the hassle of reflashing and re-configuring.
The Raspberry Pi people recommend against this approach though (or at least did last I checked), and they will probably know best about potential pitfalls in their heavy hardware specific fork of Debian, thus I won't tell people to do that on a RaspberryPiOS based image like OctoPi in the FAQ, claiming I know better than the maintainers of the underlying distro.
It definitely is a valid option on most other distros & distro variants.