As far as the "splitting at the braces thing" goes: I would not rely on this not changing. Back when I wrote the original f-string tokenizer, this was just an optimization so I could play C games with pointers to null-terminated strings. I'd temporarily change "this: {{braces}} done" to be these strings, in turn:

"this: {\0"
"braces}\0"
" done\0"

After I was done, I'd replace the '\0' with whatever was originally there. Doing it this way, I didn't have to allocate space for a new string without the doubled braces. I can easily image a future where this tradeoff changes, or is only used for strings longer than some temporary fixed size buffer, or something like that.

I don't know if the PEP 701 tokenizer kept this behavior deliberately for compatibility, or if it was just easier for them, too.

For the rest of your questions: @pablogsal