I take it back. The 4-value state looks better.

My initial hesitance was that if you ever see GEN_RUNNING you are
probably already in trouble, since you can't call send, next, throw or
even close on a running generator (they all throw ValueError), so why
are you looking at its state at all? But most reasons for looking at
the state are obscure anyway, and from a different perspective it's a
nice state machine. (Note that there's no transition from SUSPENDED to
CLOSED -- you have to go through RUNNING to possibly handle
GeneratorExit.)