Here is the recipe again. Previous mail was sent using Gnus
through GMail. I usually have no troubles. I suppose the problem
came from inserted diff output; Thus this time I attached the diff
as a MIME part.

> Could you provide a step-by-step recipe, starting from "emacs
> -Q"?

Steps to reproduce (I am now using GNU Emacs 28.0.50 (build 29,
x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo version 1.16.0) of
2020-11-21), commit 789ee3e1d5 from master:

1. M-x find-file /usr/lib/python3.7/copy.py RET  2. C-x C-w
~/test.py RET  3. C-v C-v C-u 15 C-n <= scroll two pages and reach
an blank line where inserting a new class definition makes sense
4. M-x python-skeleton-class  6. C-M-S-v  7. C-M-S-v  8. Test <= I
wrote this in the minibuffer where I was prompted since step 4 for
a class name as required by python-skeleton-class  7. Press RET 

Now the diff of the resulting file is the following. As you can
see it's not the expected result of python-skeleton-class.   



Feel free to ask if it's still unclear.   Regards,

--
Matthias