The problem Richard describes isn´t a lost wakeup.  PyCOND_SIGNAL _will_ wake up _at least_ one thread.  It just isn't guaranteed to be one of those who previously called PyCOND_WAIT(): It could be a latecomer to the game, including the one who called Signal himself.  If no such thread comes in to steal it, then one of the waiting threads _will_ wake up.

None of the internal usages of condition variables makes this assumption about the order of wakeup from PyCOND_WAIT().