Terry J. Reedy <tjreedy@udel.edu> added the comment: The official line is the claim in the docs that tkinter *is* threadsafe, repeated by Martin on at least one issue. See the first paragraph above. Therein lies the problem. A reason to not just reverse the claim is that it is, at least for practical purposes, true, by default, for 8.6. See the opening post of #11077 for one motivation for calling from threads. (Numerous others have tried and reported failures, though usually without explaining why they tried.) Mark, do you know how to identify the tcl/tk build on MacOS or Linux? If so, and you have installed 3.7.0 on MacOS since b0, which includes 8.6.8, can you report which it is? Ditto for any Linux distribution you have. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue33479> _______________________________________