Use deadsnakes PPA on Ubuntu hirsute

Today I upgraded a computer of mine from Ubuntu Groovy Gorilla (20.10) to Hirsute Hippo (21.04). The process was mostly painless, but I had to go through the standard process of evaluating each file in /etc/apt/sources.d/*.distUpgrade to determine how all of my PPAs or other software repositories needed to be updated. The one that took the most work was the Deadsnakes PPA.

Deadsnakes is a truly wonderful repository containing builds of alternative Python versions. Each recent Ubuntu version ships one Python 3 version that is the system default – currently version 3.9 on Hirsute. However, if you develop Python tools or libraries, you may want alternate versions around for testing. For instance, some projects of mine have Tox files to automatically run tests on Python 3.6-3.10. The Deadsnakes repo provides several recent Python 3 releases, except the system version for your OS. However, only LTS releases are supported.

This makes sense: the majority of Ubuntu installs are LTS, so the cost/benefit tradeoff doesn’t make sense to support the more frequent 6-monthly upgrades. But I’m used to running Arch Linux and having very up-to-date packages, so the idea of staying on an LTS version with packages that are already at least 1-2 years out of date is… not pleasant. So I stick with the regular 6-monthly Ubuntu releases, and I generally make do by installing packages from repos which are targeted at the last LTS release. Surprisingly, this works quite well¹. For example, on Groovy, I was able to just edit my Deadsnakes sources.list.d file to look for packages for focal (the prior version).

Unfortunately, this doesn’t work for Hirsute. While Focal and Groovy both have the same system Python (3.8), Hirsute upgrades it to 3.9. If I installed the Deadsnakes Focal Python packages, then the python3.9 package within the PPA could replace the default Ubuntu system python – which is not ideal. Plus, since the Focal repository omits python3.8 in order to avoid replacing the system Python on Focal, I wouldn’t be able to get that version!

There is a way to get the best of both worlds though. Apt (the Ubuntu package manager) has a directory where you can place “preferences” that help prioritize (or “pin”) packages on your system. With it, you can do the following:

1. Block all packages from the Deadsnakes PPA from being installed.
2. Re-enable packages which match a particular Python version.

So, if you add the Deadsnakes Focal repository to your system, you could use this approach to ensure that the Python 3.9 package can’t be installed. But, you’d still need to find a way to install Python 3.8. You can manage this by simply going back one more Ubuntu release, to Bionic. The default Python there was 3.6, so there is a 3.8 package available to install. Thus, the final approach is:

1. Add an entry for Focal and Bionic Deadsnakes repositories within /etc/apt/sources.list.d. This entry looks something like this, and can be saved in any filename within the directory (you may already have a relevant one if you used add-apt-repository to add the PPA – in this case, just modify it).

   deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu/ focal main
   deb http://ppa.launchpad.net/deadsnakes/ppa/ubuntu/ bionic main
   

2. Add the following rules which blacklist all packages from the repos, and then selectively enable Python versions from the correct locations. This can be put in any filename within the directory /etc/apt/preferences.d/

   Explanation: Prevent installing from deadsnakes repo.
   Package: *
   Pin: release o=LP-PPA-deadsnakes
   Pin-Priority: 1
   
   Explanation: Allow installing python 3.{6,7} from deadsnakes/focal
   Package: *python3.6* *python3.7*
   Pin: release o=LP-PPA-deadsnakes,n=focal
   Pin-Priority: 500
   
   Explanation: Allow installing python 3.8 from deadsnakes/bionic
   Package: *python3.8*
   Pin: release o=LP-PPA-deadsnakes,n=bionic
   Pin-Priority: 500
   

3. Run sudo apt update to update your cached package info, and then you should be able to install other Python versions! You can use sudo apt policy to view the rules, and sudo apt policy python3.7 (for example) to see the different package versions your system is choosing from, along with their priorities.

So, is this a particularly safe setup? Well, I haven’t had too much trouble. It’s definitely not supported – so you shouldn’t bug the Deadsnakes maintainers with issues that spring up if you try this approach. If you’re familiar with the tooling (read through man apt_preferences) and comfortable poking around if things get a bit messed up, then you should have no problem using this as a daily driver.

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