I've made some changes to what Ivan started, which you can find here: https://github.com/roseman/cpython/tree/tkinter_docs

The first two commits are minor updates/improvements not really related to threading, and I suspect are uncontroversial. 

The last commit rewrites the thread model section. The main focus is on how things work in practice, and to eliminate what I previously referred to as "fighting" between the two models. 

It explains how Python and Tcl differ on threads, the mappings between threads and interpreters, etc.

It plays down the comparison to other GUI toolkit event models. It still highlights the difference for people familiar with them. It doesn't try to teach both models for those who don't know either (which includes those who may not have done much if any GUI programming) since that is extraneous to their present needs. It talks simply about why event handlers blocking is a bad thing. It avoids what I think are unnecessary details about Tcl in this context.

There is then an area for special cases that highlight the various actual trouble spots when it comes to threads, what can cause them, and how to avoid them. 

I hope this covers the real issues without overly complicating things. Any thoughts?