classification
Title: Python3 allows mixture of tabs and spaces for indentation
Type: behavior Stage: resolved
Components: Interpreter Core Versions: Python 3.5
process
Status: closed Resolution: duplicate
Dependencies: Superseder: TabError behavior doesn't match documentation
View: 24260
Assigned To: Nosy List: Mikko Rantalainen, eric.smith, serhiy.storchaka, steven.daprano, xtreak
Priority: normal Keywords:

Created on 2019-10-16 08:00 by Mikko Rantalainen, last changed 2019-10-16 11:32 by eric.smith. This issue is now closed.

Messages (6)
msg354779 - (view) Author: Mikko Rantalainen (Mikko Rantalainen) Date: 2019-10-16 08:00
Contrary to PEP-8 (https://www.python.org/dev/peps/pep-0008/#tabs-or-spaces) claiming "Python 3 disallows mixing the use of tabs and spaces for indentation", in reality Python 3 allows mixing tabs and spaces without warnings or errors. For example (with all spaces replaced with middle dot and all spaces replaced with a short arrow):

#!/usr/bin/python3
for·i·in·range(2):
··for·j·in·range(3):
➧··for·k·in·range(4):
·➧···print(i,j,k,sep=",")

Either the PEP-8 should be fixed or the parser/compiler should be fixed. 

The current behavior seems a bit unstable because the exact order of spaces and tabs causes "TabError: inconsistent use of tabs and spaces in indentation" to appear more or less random.

I'd prefer python3 to require that *all whitespace* at the start of the all the lines is tabs for the whole file or spaces for the whole file. And first indented line sets the preference for the whole file.

(Personally I'd prefer tabs contrary to PEP-8 language but this bug is not about the preference. This is PEP-8 claiming things that are not true.)

$ python3 --version
Python 3.5.2
msg354780 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2019-10-16 08:05
I can verify this on 3.7.4.
msg354788 - (view) Author: Steven D'Aprano (steven.daprano) * (Python committer) Date: 2019-10-16 08:57
I believe that the interpreter only requires that each block is consistent, not that all blocks in a module are consistent.
msg354791 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-16 09:25
Thank you for your report Mikko. Yes, the heuristic used in Python 3 is unable to catch all inconsistencies in using tabs and spaces.

Such bug was already reported early. There should be an open issue.
msg354795 - (view) Author: Karthikeyan Singaravelan (xtreak) * (Python committer) Date: 2019-10-16 11:30
Seems related issue24260
msg354796 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2019-10-16 11:32
Thanks, Serhiy. This has been reported in 24260, so I'm closing this issue.
History
Date User Action Args
2019-10-16 11:32:00eric.smithsetstatus: open -> closed
superseder: TabError behavior doesn't match documentation
messages: + msg354796

type: behavior
resolution: duplicate
stage: resolved
2019-10-16 11:30:15xtreaksetnosy: + xtreak
messages: + msg354795
2019-10-16 09:25:58serhiy.storchakasetnosy: + serhiy.storchaka
messages: + msg354791
2019-10-16 08:57:31steven.dapranosetnosy: + steven.daprano
messages: + msg354788
2019-10-16 08:05:23eric.smithsetnosy: + eric.smith
messages: + msg354780
2019-10-16 08:00:10Mikko Rantalainencreate