Issue 37810: ndiff reports incorrect location when diff strings contain tabs

Here's an example

from difflib import ndiff


def main():
    x = '\tx\t=\ty\n\t \t \t^'
    y = '\tx\t=\ty\n\t \t \t^\n'
    print('\n'.join(
        line.rstrip('\n') for line in
        ndiff(x.splitlines(True), y.splitlines(True)))
    )


if __name__ == '__main__':
    exit(main())


Current output:

$ python3.8 t.py
  	x	=	y
- 	 	 	^
+ 	 	 	^
? 	     +


Expected output:

$ ./python t.py
  	x	=	y
- 	 	 	^
+ 	 	 	^
? 	 	 	 +


Found this while implementing a similar thing for flake8 here: https://gitlab.com/pycqa/flake8/merge_requests/339 and while debugging with pytest

This seems like a reasonable suggestion to me.  Am not sure whether it should be backported.

Tim, what do you think?

That's actually a good point, I don't think this should land in python3.7 since it changes outuput -- I'm removing that from the versions (though the bug does affect every version I have access to)

assignee: tim.peters
components: + Library (Lib)