Making a Persistent object Handler

A handler is a convenient way to group functionality on an object. This allows you to logically group all actions related to that thing in one place. This tutorial expemplifies how to make your own handlers and make sure data you store in them survives a reload.

For example, when you do obj.attributes.get("key") or obj.tags.add('tagname') you are evoking handlers stored as .attributes and tags on the obj. On these handlers are methods (get() and add() in this example).

Base Handler example

Here is a base way to set up an on-object handler:

from evennia import DefaultObject, create_object
from evennia.utils.utils import lazy_property

class NameChanger:
    def __init__(self, obj):
        self.obj = obj

    def add_to_key(self, suffix):
        self.obj.key = f"self.obj.key_{suffix}"

# make a test object
class MyObject(DefaultObject):
    def namechange(self):
       return NameChanger(self)

obj = create_object(MyObject, key="test")
>>> "test"
>>> "test_extra"

What happens here is that we make a new class NameChanger. We use the @lazy_property decorator to set it up - this means the handler will not be actually created until someone really wants to use it, by accessing obj.namechange later. The decorated namechange method returns the handler and makes sure to initialize it with self - this becomes the obj inside the handler!

We then make a silly method add_to_key that uses the handler to manipulate the key of the object. In this example, the handler is pretty pointless, but grouping functionality this way can both make for an easy-to-remember API and can also allow you cache data for easy access - this is how the AttributeHandler (.attributes) and TagHandler (.tags) works.

Persistent storage of data in handler

Let’s say we want to track ‘quests’ in our handler. A ‘quest’ is a regular class that represents the quest. Let’s make it simple as an example:

# for example in mygame/world/

class Quest:

    key = "The quest for the red key"

    def __init__(self):
        self.current_step = "start"

    def check_progress(self):
        # uses self.current_step to check
        # progress of this quest
        getattr(self, f"step_{self.current_step}")()

    def step_start(self):
        # check here if quest-step is complete
        self.current_step = "find_the_red_key"
    def step_find_the_red_key(self):
        # check if step is complete
        self.current_step = "hand_in_quest"
    def step_hand_in_quest(self):
        # check if handed in quest to quest giver
        self.current_step = None  # finished

We expect the dev to make subclasses of this to implement different quests. Exactly how this works doesn’t matter, the key is that we want to track self.current_step - a property that should survive a server reload. But so far there is no way for Quest to accomplish this, it’s just a normal Python class with no connection to the database.

Handler with save/load capability

Let’s make a QuestHandler that manages a character’s quests.

# for example in the same mygame/world/

class QuestHandler:
    def __init__(self, obj):
        self.obj = obj
        self.do_save = False

    def _load(self): = self.obj.attributes.get(
            "quest_storage", default={}, category="quests")

    def _save(self):
            "quest_storage",, category="quests")
        self._load()  # important
        self.do_save = False

    def add(self, questclass):[questclass.key] = questclass(self.obj)

    def check_progress(self):
        if self.do_save:
            # .do_save is set on handler by Quest if it wants to save progress

The handler is just a normal Python class and has no database-storage on its own. But it has a link to .obj, which is assumed to be a full typeclased entity, on which we can create persistent Attributes to store things however we like!

We make two helper methods _load and _save that handles local fetches and saves storage to an Attribute on the object. To avoid saving more than necessary, we have a property do_save. This we will set in Quest below.

Note that once we _save the data, we need to call _load again. This is to make sure the version we store on the handler is properly de-serialized. If you get an error about data being bytes, you probably missed this step.

Make quests storable

The handler will save all Quest objects as a dict in an Attribute on obj. We are not done yet though, the Quest object needs access to the obj too - not only will this is important to figure out if the quest is complete (the Quest must be able to check the quester’s inventory to see if they have the red key, for example), it also allows the Quest to tell the handler when its state changed and it should be saved.

We change the Quest such:

from evennia.utils import dbserialize

class Quest:

    def __init__(self, obj):
        self.obj = obj
        self._current_step = "start"

    def __serialize_dbobjs__(self):
        self.obj = dbserialize.dbserialize(self.obj)

    def __deserialize_dbobjs__(self):
        if isinstance(self.obj, bytes):
            self.obj = dbserialize.dbunserialize(self.obj)

    def questhandler(self):
        return self.obj.quests

    def current_step(self):
        return self._current_step

    def current_step(self, value):
        self._current_step = value
        self.questhandler.do_save = True  # this triggers save in handler!

    # [same as before]

The Quest.__init__ now takes obj as argument, to match what we pass to it in QuestHandler.add. We want to monitor the changing of current_step, so we make it into a property. When we edit that value, we set the do_save flag on the handler, which means it will save the status to database once it has checked progress on all its quests. The Quest.questhandler property allows to easily get back to the handler (and the object on which it sits).

The __serialize__dbobjs__ and __deserialize_dbobjs__ methods are needed because Attributes can’t store ‘hidden’ database objects (the Quest.obj property. The methods help Evennia serialize/deserialize Quest propertly when the handler saves it. For more information, see Storing Single objects in the Attributes

Tying it all together

The final thing we need to do is to add the quest-handler to the character:

# in mygame/typeclasses/

from evennia import DefaultCharacter
from evennia.utils.utils import lazy_property
from .world.quests import QuestHandler  # as an example

class Character(DefaultCharacter):
    # ...
    def quests(self):
        return QuestHandler(self)

You can now make your Quest classes to describe your quests and add them to characters with


and can later do


and be sure that quest data is not lost between reloads.

You can find a full-fledged quest-handler example as EvAdventure quests contrib in the Evennia repository.