Python is one of the programming languages that I usually edit in GNU Emacs. These days, that means using the Language Server protocol through lsp-mode and the pyls Python language server. Back when I first set this up and wrote early notes on using LSP-based editing of Python, I had not solved the problem of wanting to edit both Python 3 and Python 2 based code in my GNU Emacs sessions.

If you want to do this, it turns out to be important to run either the Python 2 pyls or the Python 3 pyls, depending on whether the file you're editing is written in Python 2 or Python 3. This creates several problems that I had to solve, and eventually did with brute force (if I used Python virtual environments, it probably would be easier).

First, it's obviously necessary to install both versions at once. I install pyls into $HOME/.local/ by using pip's '--user' switch, so I created a .local/bin/py2-pyls subdirectory and manually moved the Python 2 version of pyls from .local/bin into it. This requires me to always update the Python 2 version of pyls before the Python 3 version, which is a bit annoying, but that's life in a world of both versions.

To pick the right version of pyls to run, I use a cover script; the cover script uses various heuristic checks to try to figure out if it's being run in a directory with Python 2 or Python 3 code (it doesn't work for the case of mixed code and I'm not sure that would work in general anyway). I don't have $HOME/.local/bin on my $PATH and GNU Emacs will conveniently just try to run 'pyls' to start the Python LSP server, so I put the cover script in my $HOME/bin.

The most important checks the cover script uses are to look for which version of Python seems to be being run by '#!' lines in any *.py files in the current directory, and whether there are any obvious 'print' statements (which indicate Python 2). If you're going to do this, note that some Python programs are installed with their '#!' line being '#!/usr/bin/env python3' or the like, instead of directly running the Python interpreter. I missed this in the first version of the cover script because all of our Python scripts directly use '#!/usr/bin/python3'.

(Also, the default Python version for my cover script is Python 3, because Python 3 is what I'm writing all my new Python code in.)

All of this is basically a hack but it works pretty well for me, especially in combination with how I'm dealing with my Python indentation problem. The result is a pretty seamless LSP-based Python editing experience in GNU Emacs where everything basically works. I'm not sure I'm sold on the whole LSP-based experience for editing Python, but that's not the fault of my hacks.