class Dst:
    def __init__(self):
        self.messages = []
        self.checklist = {
            "from": None,
            "to": None,
            "time": None,
            "passangerCount": None,
            "discountedCount": None,
            "seatType": None,
            "wagonType": None
        }
        self.history = []
    
    def store(self, message):
        self.messages.append(message)

    def get_messages(self):
        return self.messages

    def get_next_question(self):
        for key, value in self.checklist.items():
            if value == None:
                return key

    def save_answer(self, slots):
        for slot in slots:
            if slot[1] is not None:
                self.checklist[slot[0]] = slot[1]

        self.messages.append(slots)