NPCs that listen to what is said

> say hi 
You say, "hi"
The troll under the bridge answers, "well, well. Hello."

This howto explains how to make an NPC that reacts to characters speaking in their current location. The principle applies to other situations, such as enemies joining a fight or reacting to a character drawing a weapon.

# mygame/typeclasses/npc.py

from characters import Character

class Npc(Character):
    """
    A NPC typeclass which extends the character class.
    """
    def at_heard_say(self, message, from_obj):
        """
        A simple listener and response. This makes it easy to change for
        subclasses of NPCs reacting differently to says.       

        """ 
        # message will be on the form `<Person> says, "say_text"`
        # we want to get only say_text without the quotes and any spaces
        message = message.split('says, ')[1].strip(' "')

        # we'll make use of this in .msg() below
        return f"{from_obj} said: '{message}'"

We add a simple method at_heard_say that formats what it hears. We assume that the message that enters it is on the form Someone says, "Hello", and we make sure to only get Hello in that example.

We are not actually calling at_heard_say yet. We’ll handle that next.

When someone in the room speaks to this NPC, its msg method will be called. We will modify the NPCs .msg method to catch says so the NPC can respond.

# mygame/typeclasses/npc.py

from characters import Character
class Npc(Character):

    # [at_heard_say() goes here]

    def msg(self, text=None, from_obj=None, **kwargs):
        "Custom msg() method reacting to say."

        if from_obj != self:
            # make sure to not repeat what we ourselves said or we'll create a loop
            try:
                # if text comes from a say, `text` is `('say_text', {'type': 'say'})`
                say_text, is_say = text[0], text[1]['type'] == 'say'
            except Exception:
                is_say = False
            if is_say:
                # First get the response (if any)
                response = self.at_heard_say(say_text, from_obj)
                # If there is a response
                if response != None:
                    # speak ourselves, using the return
                    self.execute_cmd(f"say {response}")   
    
        # this is needed if anyone ever puppets this NPC - without it you would never
        # get any feedback from the server (not even the results of look)
        super().msg(text=text, from_obj=from_obj, **kwargs) 

So if the NPC gets a say and that say is not coming from the NPC itself, it will echo it using the at_heard_say hook. Some things of note in the above example:

• Line 15 The text input can be on many different forms depending on where this msg is called from. If you look at the code of the ‘say’ command you’d find that it will call .msg with ("Hello", {"type": "say"}). We use this knowledge to figure out if this comes from a say or not.

• Line 24: We use execute_cmd to fire the NPCs own say command back. This works because the NPC is actually a child of DefaultCharacter - so it has the CharacterCmdSet on it! Normally you should use execute_cmd only sparingly; it’s usually more efficient to call the actual code used by the Command directly. For this tutorial, invoking the command is shorter to write while making sure all hooks are called

• Line26: Note the comments about super at the end. This will trigger the ‘default’ msg (in the parent class) as well. It’s not really necessary as long as no one puppets the NPC (by @ic <npcname>) but it’s wise to keep in there since the puppeting player will be totally blind if msg() is never returning anything to them!

Now that’s done, let’s create an NPC and see what it has to say for itself.

reload
create/drop Guild Master:npc.Npc

(you could also give the path as typeclasses.npc.Npc, but Evennia will look into the typeclasses folder automatically so this is a little shorter).

> say hi
You say, "hi"
Guild Master says, "Anna said: 'hi'"

Assorted notes

There are many ways to implement this kind of functionality. An alternative example to overriding msg would be to modify the at_say hook on the Character instead. It could detect that it’s sending to an NPC and call the at_heard_say hook directly.

While the tutorial solution has the advantage of being contained only within the NPC class, combining this with using the Character class gives more direct control over how the NPC will react. Which way to go depends on the design requirements of your particular game.