I'm iterating on all threads and getting its thread id to find out the thread state (in my use case) and then doing what you just did there...

So, while this solution does work for me, if the idea is making tstate opaque, then having (an optional) thread id in settrace (which iterates to find the proper thread if given) could solve it without having to rely on any CPython internals on my side (although it should probably return a bool to say if it did work then).