EditorWindow.py has this function, applied to editor and shell windows, which is obsolete for Python3 and unicode identifiers.

def fixwordbreaks(root):
    # Make sure that Tk's double-click and next/previous word
    # operations use our definition of a word (i.e. an identifier)
    tk = root.tk
    tk.call('tcl_wordBreakAfter', 'a b', 0) # make sure word.tcl is loaded
    tk.call('set', 'tcl_wordchars', '[a-zA-Z0-9_]')
    tk.call('set', 'tcl_nonwordchars', '[^a-zA-Z0-9_]')

Double clicking selects a contiguous sequence of 'word' or
'nonword' characters.
"Control-backspace deletes word left, Control-DEL deletes word right."
"Control-left/right Arrow moves by words in a strange but useful way."

It might be more useful if the REs were expanded.

I think it is enough to get rid of this function.

Deleting the function and calls to it would be easy. But my memory, which could be off, is that I left fixwordbreaks(root) in the test function now called _editor_window (at the end of the file) because the test did not work right without it. I willhave to recheck.

Beyond that, my first experiments were aimed at discovering the functions affected and therefor what would have to be tested with any changes.

To properly test this requires simulating keystrokes, like control-backspace, as opposed to inserting characters. Are there any tk/tkinter tests that do this, that I could use as a model?

word.tcl in Tcl library contains following lines:

if {$::tcl_platform(platform) eq "windows"} {
    # Windows style - any but a unicode space char
    set ::tcl_wordchars {\S}
    set ::tcl_nonwordchars {\s}
} else {
    # Motif style - any unicode word char (number, letter, or underscore)
    set ::tcl_wordchars {\w}
    set ::tcl_nonwordchars {\W}
}

So by default all works as expected in Motif style, but not in Windows style.

If you want to have same behavior in both styles, defines word chars as:

    tk.call('set', 'tcl_wordchars', r'\w')
    tk.call('set', 'tcl_nonwordchars', r'\W')

GUI tests are not needed, it is enough to test relevant Tcl commands: tcl_wordBreakAfter, tcl_wordBreakBefore, tcl_endOfWord, tcl_startOfNextWord, and tcl_startOfPreviousWord or TextSelectTo. It's interesting, there are no tests for these functions in Tcl test suite.

Now-closed duplicate bpo-33386 reported that μ, 0x3bc, is not selected as part of identifiers when double clicking. This prompted some research.

The 'Windows' style imitates the behavior of Command Prompt, which I presume is a carryover from DOS days. PowerShell stuck with it, but Notepad, Notepad++, Microsoft Word, Firefox, Thunderbird, and ??? have not. I think Tcl should have switched long ago. In any case, I will go with whatever the tcl re engine defines as word chars, the 'Motif' style', rather than attempt to write a giant re, which would have to change as characters are added.

Do we still need this line in fixwordbreaks?
tk.call('tcl_wordBreakAfter', 'a b', 0) # make sure word.tcl is loaded
I will leave it until you say we don't.

After patching, 'abcμμμdef' ('0x3bc'*3) is selected as one word instead of word, nonword, word. 'abc+efg' is still selected in 3 pieces, instead of the 1 word seen by Command Prompt.

New changeset 5ff3a16 by Terry Jan Reedy in branch 'master':
bpo-21474: Update IDLE word/identifier definition from ascii to unicode. (GH-6643)
5ff3a16

Do we still need this line in fixwordbreaks?

Yes. Loading word.tcl sets tcl_wordchars and tcl_nonwordchars. We should ensure that word.tcl is loaded and these variables are set before we change them.

New changeset 887b5f8 by Miss Islington (bot) in branch '3.7':
bpo-21474: Update IDLE word/identifier definition from ascii to unicode. (GH-6643)
887b5f8

Thank you for the help. My immediate goal for IDLE is to fix unicode problems, to the extent allowed by tk, before 3.7.0.

New changeset 3d11630 by Miss Islington (bot) in branch '3.6':
bpo-21474: Update IDLE word/identifier definition from ascii to unicode. (GH-6643)
3d11630