Fix Emacs python-mode REPL and org code block with python evaluation problems

For a while now, the python shell in Python mode in Emacs has not worked properly on my work laptop. I get some strange glyphs in the top of the python shell buffer when opening it from a file with C-c C-p and when I tried importing a file with C-c C-c, I would get an error saying stringp was nil and every character in what I tried to import was written first once, then together with the next, then the next three and so on. It looked somewhat psychedelic and was more or less unusable.

I also had trouble evaluating python code blocks in org mode which has been annoying when presenting with inter-present-mode for my students or exporting from org files I used for presentations to PDF or docx to upload to our school's learning platform (it's learning). In org, I would get the same error saying stringp was nil when evaluating code blocks that I would get when trying to import files to the REPL in python mode.

I first thought the problem was related to my einar-python-virtualenv function which locally changes the python-shell-interpreter for a file if it is in a project with a virtual environment since I made that quite recently, but the strange thing was that it worked perfectly on GNU Guix on my main machine at home. I then thought maybe it was a Windows 11 problem since I only experienced it at work, so I tried on a Raspberry Pi 4 at work with the latest Raspberry Pi OS and I had the same problem.

I then tried launching Emacs with -q to see if the problem was related to my config or not. When I then launched the python shell from a file with C-c C-p, the REPL itself worked slightly better, but I still got some strange glyphs at the top. I then spent some time trying to figure out where in my config the problem appeared and removed first the einar-python-virtualenv and then my configuration for python-mode, but the problem persisted. I also looked into whether some setting for comint-mode might cause the problem.

After spending one and a half hour trying to troubleshoot this on Monday, I today realised I updated Python to 3.14 on my work Windows 11 laptop a while ago. I checked the python version on the Raspberry Pi and it was 3.13. I checked packages.guix.gnu.org and Guix uses version 3.11. I thought maybe something changed in one of the newer versions. I had a look at news for version 3.14 and did not find anything promising. I then checked out what was new in 3.13 and one of the first things was a new and improved python shell.

Luckily, you can get the old shell that python mode in Emacs works with by setting the environment variable PYTHON_BASIC_REPL to something. When I added this function call to my Emacs config for python mode, the python REPL works in python mode again and it is possible to evaluate org code blocks and export files that will evaluate them on export again:

(setenv "PYTHON_BASIC_REPL" "1")

I write this so others experiencing the same problem can find a solution. In the long term, python-mode, python-ts-mode and org mode needs to work with the new and improved python shell since the old shell will probably be removed at some point in the future, but for now, just setting the environment variable is enough to get both the REPL and org code blocks working again.