If a fixed set of slots cannot meet your needs in a form, we strongly recommend using the Required Slots feature, which allows dynamic changes to the slots required in a form according to the user’s input within the form. Here is an example.

user: I'd like to book a chinese restaurant.
bot: Do you want to sit outside?
user: Yes / No
bot: Do you want to sit in the shade or in the sun? / Please provide additional preferences

This is a conversation between a user and a restaurant booking assistant. The assistant asks the user to fill the slot seat. If the user responds ‘yes’, the bot needs to inquire about the details. However, if the user replies ‘no’, the bot will not collect the seat preference slot. We can realize this conversation by writing code below.

class ValidateRestaurantForm(FormValidationAction):
    def name(self) -> Text:
        return "validate_restaurant_form"

    async def required_slots(
        self,
        domain_slots: List[Text],
        dispatcher: "CollectingDispatcher",
        tracker: "Tracker",
        domain: "DomainDict",
    ) -> List[Text]:
        additional_slots = ["outdoor_seating"]
        if tracker.slots.get("outdoor_seating") is True:
            # If the user wants to sit outside, ask
            # if they want to sit in the shade or in the sun.
            additional_slots.append("shade_or_sun")

        return additional_slots + domain_slots

The required_slots function returns a list of slots that the form needs to collect. Before the chatbot asks for filling any slot, rasa will execute the required_slots function to determine which slots should be collected. Consequently, we can use the tracker.get_slot function to obtain previously filled slots, and then return the slots that need to be collected. Note that if you need not change required slots dynamically, you need not touch this function. PromptDialog will automatically generate the required slots based on the form diagram.

The feature is available when you click the Slots node and then choose Edit Node. It will be displayed on the right panel.