structure by indentation … grrrr ….

If you have to do this:

:%s/\t/    /g

in order to get a very simple function to compile because of this error

  File "./", line 13
    return sum
IndentationError: unindent does not match any outer indentation level

even though your editor (atom!!!!??!?!) wasn’t showing you these mixed tabs and spaces … Yeah, there is something profoundly wrong with the approach.
The function in question was all of 10 lines. The error was undetectable in the atom editor. Vim saves the day yet again, but … it … shouldn’t … have … to …