Our long time custom for installing Django for our Django based web application was to install it with 'python3 setup.py install --prefix /some/where', and then set a PYTHONPATH environment variable that pointed to /some/where/lib/python<ver>/site-packages. Up through at least Python 3.10 (in Ubuntu 22.04), you could start Python 3 and then successfully do 'import django' with this; in fact, it worked on different Python versions if you were pointing at the same directory tree (in our case, this directory tree lives on our NFS fileservers). In our Ubuntu 24.04 version of Python 3.12 (which also has the Ubuntu packaged setuptools installed), this no longer works, which is inconvenient to us.

(It also doesn't seem to work in Fedora 40's 3.12.8, so this probably isn't something that Ubuntu 24.04 broke by using an old version of Python 3.12, unlike last time.)

The installed site-packages directory contains a number of '<package>.egg' directories, a site.py file that I believe is generic, and an easy-install.pth that lists the .egg directories. In Python 3.10, strace says that Python 3 opens site.py and then easy-install.pth during startup, and then in a running interpreter, 'sys.path' contains the .egg directories. In Python 3.12, none of this happens, although CPython does appear to look at the overall 'site-packages' directory and 'sys.path' contains it, as you'd expect. Manually adding the .egg directories to a 3.12 sys.path appears to let 'import django' work, although I don't know if everything is working correctly.

I looked through the 3.11 and 3.12 "what's new" documentation (3.11, 3.12) but couldn't find anything obvious. I suspect that this is related to the removal of distutils in 3.12, but I don't know enough to say for sure.

(Also, if I use our usual Django install process, the Ubuntu 24.04 Python 3.12 installs Django in a completely different directory setup than in 3.10; it now winds up in <top level>/local/lib/python3.12/dist-packages. Using 'pip install --prefix ...' does create something where pointing PYTHONPATH at the 'dist-packages' subdirectory appears to work. There's also 'pip install --target', which I'd forgotten about until I stumbled over my old entry.)

All of this makes it even more obvious to me than before that the Python developers expect everyone to use venvs and anything else is probably going to be less and less well supported in the future. Installing system-wide is probably always going to work, and most likely also 'pip install --user', but I'm not going to hold my breath for anything else.

(On Ubuntu 24.04, obviously we'll have to move to a venv based Django installation. Fortunately you can use venvs with programs that are outside the venv.)