So I had a thing happen :

This is my angry face that GNU Emacs appears to have re-indented my entire Python file to a different standard without me noticing and I didn't catch it in time. And also it appears impossible in GNU Emacs to FIX this. I do not want four space no tabs, this is historical code that all files should be eight spaces with tabs (yes, Python 2).

That 'Python 2' bit turns out to be load-bearing. The specific problem turned out to be that if I hit TAB with a region selected or M-q when GNU Emacs point was outside a comment, the entire file was reformatted to modern 4-space indents (and long expressions got linewrapped, and some other formatting changes). I'm not sure which happened to trigger the initial reformatting that I didn't notice in time, but I suspect I was trying to use M-q to reflow a file level comment block and had my cursor (point) in the wrong spot. My TAB and M-q bindings are standard , and when I investigated deeply enough I discovered that this was LSP related.

The first thing I learned is that just 'turning off' LSP mode with 'lsp-mode' (or 'M-: (lsp-mode -1))' isn't enough to actually turn off LSP based indentation handling. This is discussed in lsp-mode issue #824 , and apparently the solution is some combination of deactivating an additional minor mode, invoking lsp-disconnect through M-x (or using the 's-l w D' key binding if you have Super available), or setting lsp-enable-indentation to 'nil' (probably as a buffer-local variable, although tastes may differ).

The second thing I discovered is that in my environment this doesn't happen for Python 3 code. With my normal Python 3 GNU Emacs LSP environment, using python-lsp-server (pylsp) ( also ), the LSP environment will make no changes and report 'No formatting changes provided'. My problem only happens in Python 2 buffers, and that's because in Python 2 buffers I wasn't using pylsp (which only officially supports Python 3 code) but instead the older and now unsupported pyls . Either pyls has always behaved differently than pylsp when the LSP server asks it to do formatting stuff, or at some point the LSP protocol and expectations around formatting actions changed and pyls (which has been unmaintained since 2020) didn't change to keep up.

My immediate fix was to set lsp-enable-indentation to nil in my GNU Emacs lsp-mode hook for python-mode. As a longer term thing I'm going to experiment with using pylsp even for Python 2 code, to see how it goes. Otherwise I may wind up disabling LSP for Python 2 code and buffers, although that's somewhat tricky since there's no explicit separate settings for Python 2 versus Python 3. Another immediate fix is that in the future I may be editing this particular code base more in vi(m) or perhaps sam than GNU Emacs.

(My Python 2 code is mostly or entirely written using tabs for indentation, so the presence of leading tabs is a reliable way of detecting 'Python 2' code.)

PS: This particular Python 2 program is DWiki , the wiki engine underlying Wandering Thoughts , so while it will move to Python 3 someday and I once got a hacked version vaguely running that way , it's not going to happen any time soon for multiple reasons.