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

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

[Per Starbäck]

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.

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.

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

[Lars Ingebrigtsen]

starback@stp.lingfil.uu.se (Per Starbäck) writes:

> 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.

Andreas Röhler <andreas.roehler@easy-emacs.de> writes:

> python.el hard-codes
>
> (set (make-local-variable 'indent-tabs-mode) nil)
>
> which should be customizable.

Well, I don't know -- the problem is that mixing them leads to invalid
programs, so specifying a single tab mode here makes sense, since it
will presumably lead to fewer problems in the long run.

That is, python-mode doesn't really support .py files with tabs in
them -- as a design decision.  So I think this works as designed.

Anybody else have an opinion here?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Well, I don't know -- the problem is that mixing them leads to invalid
> programs, so specifying a single tab mode here makes sense, since it
> will presumably lead to fewer problems in the long run.
>
> That is, python-mode doesn't really support .py files with tabs in
> them -- as a design decision.  So I think this works as designed.
>
> Anybody else have an opinion here?

Nobody had in half a year, so I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no