On Oct 7, 2017 12:20, "Koos Zevenhoven" <k7hoven@gmail.com> wrote: Unfortunately, we actually need a third kind of generator semantics, something like this: @contextvars.caller_context def genfunc(): assert cvar.value is the_value yield assert cvar.value is the_value with cvar.assign(the_value): gen = genfunc() next(gen) with cvar.assign(1234567890): try: next(gen) except StopIteration: pass Nick, Yury and I (and Nathaniel, Guido, Jim, ...?) somehow just narrowly missed the reasons for this in discussions related to PEP 550. Perhaps because we had mostly been looking at it from an async angle. That's certainly a semantics that one can write down (and it's what the very first version of PEP 550 did), but why do you say it's needed? What are these reasons that were missed? Do you have a use case? -n