The change is a super great idea (see below), but I now think (my suggestion of) '`' is a bad idea because it conflicts with the use of '`' as markup for code and output, as in rst and stackoverflow.

https://stackoverflow.com/questions/37904750/tkinter-tclerror-invalid-command-name-54600176-error-what-is-going-on basically asks

What does '''_tkinter.TclError: invalid command name ".54600176"''' mean?

In my comment to the answer, I noted that 3.6.0a2 now says '''invalid command name ".`label"'''.  I wanted to write `invalid command name ".`label"` to quote it properly, SO style, but this would not work.  So I suggest we invoke your last comment and instead try '^' (or even '~' or '-' if you prefer).