Getting the Python LSP server working with venvs the brute force way

January 27, 2024

Recently I wound up doing some Django work using a Python venv, since this is the easiest way to get a self-contained Python environment that has some version of Django (or other applications) installed. However, one part of the experience was a little bit less than ideal. I normally write Python using GNU Emacs and the Python LSP server, and this environment was complaining about being unable to find Django modules to do code intelligence things with them. A little thought told me why; GNU Emacs was running my regular LSP server, which is installed through pipx into its own venv, and that venv didn't have Django installed (of course).

As far as I know, the Python LSP server doesn't have any specific support for recognizing and dealing with venvs, and in general this is a bit difficult (the version of Python being used by a venv may not even match the version that pylsp is running with; in fact this was my case, since my installed pylsp was using pypy but the Django venv was using the system Python 3). Rather than try to investigate deeply into this, I decided to solve the problem with brute force, which is to say that I installed the Python LSP server (with the right set of plugins) into the venv, along with all of the rest of things, and then ran that instance of GNU Emacs with its $PATH set to use the venv's bin/ directory and pick up everything there, including its Python 3 and python-lsp-server.

This is a little bit aesthetically displeasing for at least two reasons. First, the Python LSP server and its plugins and their dependencies aren't a small thing and anyway they're not a runtime package dependency, it's purely for development convenience. Second, the usual style of using GNU Emacs is to start it once and then reuse that single Emacs instance for everything, which naturally gives that Emacs instance a single $PATH and make it want to use a single version of python-lsp-server. I'm okay with deviating from this bit of Emacs practice, but other people may be less happy.

(A hack that deals with the second issue would be a 'pylsp' cover script that hunts through the directory tree to see if you're running it from inside a venv and if that venv has its own 'pylsp' binary; if both are true, you run that pylsp instead of your regular system-wide one. I may write this hack someday, partly so that I can stop having to remember to add the venv to my $PATH any time I want to fire up Emacs on the code I'm working on in the venv.)

Written on 27 January 2024.
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Last modified: Sat Jan 27 21:50:05 2024
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