bug#32876: 26.1; python-mode mixes existing tabs with new spaces

[Andreas Röhler]

On 29.09.2018 18:13, Per Starbäck wrote:
> In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
>
> ### Problem ###
>
> I did "emacs -Q pytab.py" where pytab.py contains the two lines:
>
> ------------------------------
> def foo():
>         print('hello')
> ------------------------------
>
> The second line begins with a TAB.
>
> Then I add a line to this marvellous program with
>
>         M-> TAB print('world') C-x C-s
>
> This new line will have eight spaces in the beginning, and because of
> that this is no longer a valid Python3 program!
>
> ### Background ###
>
> Earlier you could mix spaces and tabs (seen as 8 spaces) in Python, but
> Python3 disallows mixing them.
>
> Spaces are preferred, and the style guide "PEP 8" says that
>
>    Tabs should be used solely to remain consistent with code that is
>    already indented with tabs.
>
> So python-mode ought to help you remain consistent with such old
> code. I think that checking with what characters the current block is
> indented all the time would be overdoing it for something that shouldn't
> occur that often, but that it should suffice to just scan through the
> visited file to see if it contains any tabs in that case instead turn on
> indent-tabs-mode instead of turning it off.

May confirm that with

GNU Emacs 27.0.50 (build 1, i686-pc-linux-gnu, GTK+ Version 3.14.5) of 
2018-09-19

python.el hard-codes

(set (make-local-variable 'indent-tabs-mode) nil)

which should be customizable.