$ head 'c:\Py\Lib\lib-tk\Tix.py' -n 1
# -*-mode: python; fill-column: 75; tab-width: 8; coding: iso-latin-1-unix -*-

There's no "iso-latin-1-unix" encoding in Python, so this declaration produces an error in some code analysis tools (I have it in PyScripter), as it should according to PEP263 .

In 3.x, this was fixed in changeset d63344ba187888b6792ba8362a0dd09e06ed2f9a .

I am a little puzzled as to how a file rename changed the content, but the annotation history seems to show that.  Anyway, ...

When I load the file in IDLE 2.7, I get a warning.  I am a bit surprised as this is not a proper encoding declaration.  IDLE's re must be a bit loose.

In 3.x, the file starts with

# -*-mode: python; fill-column: 75; tab-width: 8 -*-
#
# $Id$
#

This is all ancient, obsolete, junk specific to some editor.  (The file itself not used 4 space indents.) I think it should be removed from all current versions.  As near as I can tell, there are no non-ascii chars in the file.

I'm more puzzled how noone has noticed this until now if it's supposed to produce an error upon compilation. (Well, it doesn't. I couldn't quite figure out how the encoding declaration is parsed, but it's clear the line _isn't_ matched as a regex like the docs say.)

I reread
https://docs.python.org/27/reference/lexical_analysis.html#encoding-declarations
A first or second line must be a comment matching "coding[=:]\s*([-\w.]+)" (which IDLE uses) and the captured name "must be recognized by Python".

I also did some experiments.  Apparently, "iso-latin-1-unix" is recognized by Python.  On Windows, from an IDLE editor,
  # coding: iso-latin-1-unix
runs, while 
  # coding: xiso-latin-1-unix
raises, during the compile(..., 'file', 'exec') call:
  SyntaxError: unknown encoding: xiso-latin-1-unix

Since codecs.lookup() returns the same error for both lines:
  LookupError: unknown encoding: iso-latin-1-unix
compile() must be doing something other than simply calling codecs.lookup.  I suspect it somehow recognizes 'iso', 'latin-1', and 'unix' as valid chunks of an ecoding name.  (The last might even be an obsolete legacy item.)  Whatever it is, it is not obviously available to tools written in Python.

Note that 'recognized as a legitimate encoding name' and 'available on a particular installation' are different concepts. I believe codecs.lookup implements the latter.

Serhiy, if you agree with the proposed removal, but want me to do it, I will.

Yes, CPython tokenizer recognizes encoding starting with "iso-latin-1-" as "iso-8859-1" (see get_normal_name() in Parser/tokenizer.c:228).

I agreed that coding cookie or all line can be removed from Tix.py. Please do that.

New changeset ef03aff3b195 by Terry Jan Reedy in branch '2.7':
Issue 28923: Remove editor artifacts from Tix.py,
https://hg.python.org/cpython/rev/ef03aff3b195

New changeset eb8667196f93 by Terry Jan Reedy in branch '3.5':
Issue 28923: Remove editor artifacts from Tix.py.
https://hg.python.org/cpython/rev/eb8667196f93

New changeset 4a82412a3c51 by Terry Jan Reedy in branch '3.6':
Issue 28923: Remove editor artifacts from Tix.py,
https://hg.python.org/cpython/rev/4a82412a3c51

New changeset 41031fdc924a by Terry Jan Reedy in branch 'default':
Issue 28923: Remove editor artifacts from Tix.py,
https://hg.python.org/cpython/rev/41031fdc924a