def on(self, event, cb):
        if event not in self._observers:
            self._observers[event] = [cb, ]
        else:
            if self._observers[event] not in cb:
                self._observers[event].append(cb)
            else:
                raise ValueError(
                    "Observer is already registered to event: ",
                    event)
dataset/ETHPy150Open theycallmeswift/BreakfastSerial/BreakfastSerial/util.py/EventEmitter.on/swapped operands of `not in`