Андрей Парамонов <cmr.pent@gmail.com> writes:

> Unfortunately my distrib doesn't seem to have the source el-files :-(

You can find it here: http://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/progmodes/python.el?h=emacs-25.3

Make sure to evaluate the changed source *after* you've loaded the
builtin python.el.

>     Try the recipe in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=
>     28051#8
>     and see if you suffer from that bug.
>
> Hm, the bug and the one mentioned inside describe bad behavior of
> Python2, but in my case it's the opposite :-/

Well really it describes the bad behaviour of the fallback completion.
It just happens that Emacs 25 uses the fallback completion method with
Python2 on Windows.