sitecustomize: executing code when loading python
I recently installed the version 3.9 of Python in my computer. For the moment, I want to retain the version 3.8 alongside it (probably the best way to do it would be using virtualenvs, but I don’t have time to learn it now). Obviously, some packages needed new versions: the usual suspects numpy, scipy, matplotlib, etc. But for the rest of packages, I wanted to keep the old ones, so I don’t have to manage two versions of the same packages.
PYTHONPATH
Python keeps their modules in several folders across the computer. The list of search directories can be checked with
import sys
sys.path
By default, sys.path contains the current directory, and standard folders as /usr/lib/python3.X and ~/.local/lib/python3.X/dist-packages (for Linux). Folders are checked in the order they appear on path, so Python imports the module from the first module available. During runtime, this sys.path can be modified as any normal list, appending or removing elements. Another way is using
import site
site.addsitepackages('/my/modules/folder/')
Both methods modify the search path for the current session only. For a more permanent solution, the usual suggestion is to modify the enviroment varaible $PYTHONPATH, for example in ~/.bashrc. Sounds good, right?
The problem is that the folders in $PYTHONPATH are added at the start of sys.path. In my case, python tried to import the 3.8 versions of numpy and scipy instead of their 3.9 versions, resulting in an error.
sitecustomize
The file sitecustomize.py allows to execute some code when python loads. In my computer, this file is located at /usr/lib/python3.X/sitecustomize.py, which was in fact a symlink to /etc/python3.X/sitecustomize. I modified that file to add the search directories to the end of sys.path every time that I open python3.9.
You can check that the code is really executed at the start of the python session by adding a print("Hello world") to sitecustomize.py. You’ll see the greeting at the very first line of the python session, even before the Python and GCC version numbers.
