Fix handling of mismatched Python version in virtualenv

[sbivol]

Description

This fixes #2547 for the (default) case when g:black_use_virtualenv = 1.
The fix for g:black_use_virtualenv = 0 is a superset of these changes but is more invasive so I'll create a separate branch for it.

To trigger the bug on Linux:

• make sure you don't have black installed as a system package: apt purge black
• launch vim built with Python 3.X
• run :BlackVersion, let it finish creating the virtualenv, close vim
• launch vim built with Python 3.Y
• run :BlackVersion, you should see an error: ModuleNotFoundError: No module named 'black'

I usually encounter this issue when I upgrade to a new Kubuntu release which has a newer Vim built against a newer Python, since I keep the same user data so the old virtualenv created for black is still there.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[sbivol]

I have rebased the branch, please review.

[MeGaGiGaGon]

I'm trying to use this locally, but it doesn't work for me, though I am completely inexperienced with vim, so I may have done something wrong. Here's what I did:

1. Use uv venv --python 3.12 to set up the version missmatch
2. Source the venv
3. Manually install the changed plugin via slightly altering the commands in https://black.readthedocs.io/en/latest/integrations/editors.html#vim-8-native-plugin-management
4. Run vim
5. Try :Black

What I think is happening is that it looks like black is being installed into /home/user/.vim/black/lib/python3.12/site-packages for me, while the plugin is expecting /home/user/.vim/black/lib/python3.10/site-packages, so it still doesn't work, because your changes try to find black in the venv, while the actual python used is the one bundled with vim.

Edit:

I see that this may only work with g:black_use_virtualenv = 1? But it still doesn't work for me when I run :let g:black_use_virtualenv = 1 first before :Black. It looks like /home/user/.vim/black/lib/python3.10/site-packages is still being searched, while the correct path should be the expanded version of .venv/lib/python3.12/site-packages

[sbivol]

Hi @MeGaGiGaGon,

Thank you for looking into this.

The issue addressed in this PR happens when upgrading from a VIM built against python3.12 to a VIM built against python3.13.
I wrote these as 3.X and 3.Y in the PR description but, reading it again, it wasn't the most intuitive way to highlight this difference.
So the next thing you need to do to trigger this is to start a different VIM, built against a different Python version, but keeping the same ~/.vimrc and ~/.vim/.
g:black_use_virtualenv = 1 is also a requirement to test the fix, but is not a requirement to trigger the issue (it's just that this PR does not resolve the more complicated case when g:black_use_virtualenv = 0).

Please see the list of steps below for replicating this with Docker containers.

Steps

Commands prefixed with $ run on your machine, those with © – in the container.

Create a volume to hold the shared data

$ docker volume create home

First container:

$ docker run -it --name u24-vim ubuntu:24.04 bash -c "apt-get -qq update && DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -qq install --no-install-recommends -y vim wget ca-certificates python3-venv"
$ docker commit u24-vim u24-vim
$ docker run -it -v home:/home/ubuntu/ --user ubuntu u24-vim bash
© mkdir -p ~/.vim/{autoload,plugin}
© wget -nv https://github.com/psf/black/raw/refs/heads/main/plugin/black.vim -P ~/.vim/plugin/
© wget -nv https://github.com/psf/black/raw/refs/heads/main/autoload/black.vim -P ~/.vim/autoload/
© vim -c "BlackVersion"
# Black should install successfully and display: `Black, version 25.1.0 on Python 3.12.3`.
# Quit Vim.
© exit

Second container:

$ docker run -it --name u25-vim ubuntu:25.04 bash -c "apt-get -qq update && DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get -qq install --no-install-recommends -y vim wget ca-certificates python3-venv"
$ docker commit u25-vim u25-vim
$ docker run -it -v home:/home/ubuntu/ --user ubuntu u25-vim bash
© vim -c "BlackVersion"
# You should now see a traceback that end with: `NameError: name 'Black' is not defined`.
# Exit Vim.
# Download black.vim from this PR:
© wget -nv https://raw.githubusercontent.com/psf/black/a4c4176b6e83e443901e75e5d62f880a1307c7a0/autoload/black.vim -O ~/.vim/autoload/black.vim
© vim -c "BlackVersion"
# Now you should see `Black, version 25.1.0 on Python 3.13.3 `
© exit

Back to the first container

$ docker run -it -v home:/home/ubuntu/ --user ubuntu u24-vim bash
© vim -c "BlackVersion"
# Now you should see `Black, version 25.1.0 on Python 3.12.3`

I hope this helps.

[MeGaGiGaGon]

Thank you for the clear explanation and reproduction! I think I understand the logic here fully now, so LGTM