5. Handling Equipment

In Knave, you have a certain number of inventory “slots”. The amount of slots is given by CON + 10. All items (except coins) have a size, indicating how many slots it uses. You can’t carry more items than you have slot-space for. Also items wielded or worn count towards the slots.

We still need to track what the character is using however: What weapon they have readied affects the damage they can do. The shield, helmet and armor they use affects their defense.

We have already set up the possible ‘wear/wield locations’ when we defined our Objects in the previous lesson. This is what we have in enums.py:

# mygame/evadventure/enums.py

# ...

class WieldLocation(Enum):
    
    BACKPACK = "backpack"
    WEAPON_HAND = "weapon_hand"
    SHIELD_HAND = "shield_hand"
    TWO_HANDS = "two_handed_weapons"
    BODY = "body"  # armor
    HEAD = "head"  # helmets

Basically, all the weapon/armor locations are exclusive - you can only have one item in each (or none). The BACKPACK is special - it contains any number of items (up to the maximum slot usage).

5.1. EquipmentHandler that saves

Create a new module mygame/evadventure/equipment.py.

In default Evennia, everything you pick up will end up “inside” your character object (that is, have you as its .location). This is called your inventory and has no limit. We will keep ‘moving items into us’ when we pick them up, but we will add more functionality using an Equipment handler.

A handler is (for our purposes) an object that sits “on” another entity, containing functionality for doing one specific thing (managing equipment, in our case).

This is the start of our handler:

# in mygame/evadventure/equipment.py 

from .enums import WieldLocation

class EquipmentHandler: 
    save_attribute = "inventory_slots"
    
    def __init__(self, obj): 
        # here obj is the character we store the handler on 
        self.obj = obj 
        self._load() 
        
    def _load(self):
        """Load our data from an Attribute on `self.obj`"""
        self.slots = self.obj.attributes.get(
            self.save_attribute,
            category="inventory",
            default={
                WieldLocation.WEAPON_HAND: None, 
                WieldLocation.SHIELD_HAND: None, 
                WieldLocation.TWO_HANDS: None, 
                WieldLocation.BODY: None,
                WieldLocation.HEAD: None,
                WieldLocation.BACKPACK: []
            } 
        )
    
    def _save(self):
        """Save our data back to the same Attribute"""
        self.obj.attributes.add(self.save_attribute, self.slots, category="inventory")

This is a compact and functional little handler. Before analyzing how it works, this is how we will add it to the Character:

# mygame/evadventure/characters.py

# ... 

from evennia.utils.utils import lazy_property
from .equipment import EquipmentHandler 

# ... 

class EvAdventureCharacter(LivingMixin, DefaultCharacter):
    
    # ... 

    @lazy_property 
    def equipment(self):
        return EquipmentHandler(self)

After reloading the server, the equipment-handler will now be accessible on character-instances as

character.equipment

The @lazy_property works such that it will not load the handler until someone actually tries to fetch it with character.equipment. When that happens, we start up the handler and feed it self (the Character instance itself). This is what enters __init__ as .obj in the EquipmentHandler code above.

So we now have a handler on the character, and the handler has a back-reference to the character it sits on.

Since the handler itself is just a regular Python object, we need to use the Character to store our data - our Knave “slots”. We must save them to the database, because we want the server to remember them even after reloading.

Using self.obj.attributes.add() and .get() we save the data to the Character in a specially named Attribute. Since we use a category, we are unlikely to collide with other Attributes.

Our storage structure is a dict with keys after our available WieldLocation enums. Each can only have one item except WieldLocation.BACKPACK, which is a list.

5.2. Connecting the EquipmentHandler

Whenever an object leaves from one location to the next, Evennia will call a set of hooks (methods) on the object that moves, on the source-location and on its destination. This is the same for all moving things - whether it’s a character moving between rooms or an item being dropping from your hand to the ground.

We need to tie our new EquipmentHandler into this system. By reading the doc page on Objects, or looking at the DefaultObject.move_to docstring, we’ll find out what hooks Evennia will call. Here self is the object being moved from source_location to destination:

  1. self.at_pre_move(destination) (abort if return False)

  2. source_location.at_pre_object_leave(self, destination) (abort if return False)

  3. destination.at_pre_object_receive(self, source_location) (abort if return False)

  4. source_location.at_object_leave(self, destination)

  5. self.announce_move_from(destination)

  6. (move happens here)

  7. self.announce_move_to(source_location)

  8. destination.at_object_receive(self, source_location)

  9. self.at_post_move(source_location)

All of these hooks can be overridden to customize movement behavior. In this case we are interested in controlling how items ‘enter’ and ‘leave’ our character - being ‘inside’ the character is the same as them ‘carrying’ it. We have three good hook-candidates to use for this.

  • .at_pre_object_receive - used to check if you can actually pick something up, or if your equipment-store is full.

  • .at_object_receive - used to add the item to the equipmenthandler

  • .at_object_leave - used to remove the item from the equipmenthandler

You could also picture using .at_pre_object_leave to restrict dropping (cursed?) items, but we will skip that for this tutorial.

# mygame/evadventure/character.py 

# ... 

class EvAdventureCharacter(LivingMixin, DefaultCharacter): 

    # ... 
    
    def at_pre_object_receive(self, moved_object, source_location, **kwargs): 
        """Called by Evennia before object arrives 'in' this character (that is,
        if they pick up something). If it returns False, move is aborted.
        
        """ 
        return self.equipment.validate_slot_usage(moved_object)
    
    def at_object_receive(self, moved_object, source_location, **kwargs): 
        """ 
        Called by Evennia when an object arrives 'in' the character.
        
        """
        self.equipment.add(moved_object)

    def at_object_leave(self, moved_object, destination, **kwargs):
        """ 
        Called by Evennia when object leaves the Character. 
        
        """
        self.equipment.remove(moved_object)

Above we have assumed the EquipmentHandler (.equipment) has methods .validate_slot_usage, .add and .remove. But we haven’t actually added them yet - we just put some reasonable names! Before we can use this, we need to go actually adding those methods.

5.3. Expanding the Equipmenthandler

5.4. .validate_slot_usage

Let’s start with implementing the first method we came up with above, validate_slot_usage:

# mygame/evadventure/equipment.py 

from .enums import WieldLocation, Ability

class EquipmentError(TypeError):
    """All types of equipment-errors"""
    pass

class EquipmentHandler: 

    # ... 
    
    @property
    def max_slots(self):
        """Max amount of slots, based on CON defense (CON + 10)""" 
        return getattr(self.obj, Ability.CON.value, 1) + 10
        
    def count_slots(self):
        """Count current slot usage""" 
        slots = self.slots
        wield_usage = sum(
            getattr(slotobj, "size", 0) or 0
            for slot, slotobj in slots.items()
            if slot is not WieldLocation.BACKPACK
        )
        backpack_usage = sum(
            getattr(slotobj, "size", 0) or 0 for slotobj in slots[WieldLocation.BACKPACK]
        )
        return wield_usage + backpack_usage
    
    def validate_slot_usage(self, obj):
          """
          Check if obj can fit in equipment, based on its size.
          
          """
          if not inherits_from(obj, EvAdventureObject):
              # in case we mix with non-evadventure objects
              raise EquipmentError(f"{obj.key} is not something that can be equipped.")
  
         size = obj.size
         max_slots = self.max_slots
         current_slot_usage = self.count_slots()
         return current_slot_usage + size <= max_slots:

We add two helpers - the max_slots property and count_slots, a method that calculate the current slots being in use. Let’s figure out how they work.

5.4.1. .max_slots

For max_slots, remember that .obj on the handler is a back-reference to the EvAdventureCharacter we put this handler on. getattr is a Python method for retrieving a named property on an object. The Enum Ability.CON.value is the string Constitution (check out the first Utility and Enums tutorial if you don’t recall).

So to be clear,

getattr(self.obj, Ability.CON.value) + 10

is the same as writing

getattr(your_character, "Constitution") + 10

which is the same as doing something like this:

your_character.Constitution + 10

In our code we write getattr(self.obj, Ability.CON.value, 1) - that extra 1 means that if there should happen to not be a property “Constitution” on self.obj, we should not error out but just return 1.

5.4.2. .count_slots

In this helper we use two Python tools - the sum() function and a list comprehension. The former simply adds the values of any iterable together. The latter is a more efficient way to create a list:

new_list = [item for item in some_iterable if condition]
all_above_5 = [num for num in range(10) if num > 5]  # [6, 7, 8, 9]
all_below_5 = [num for num in range(10) if num < 5]  # [0, 1, 2, 3, 4]

To make it easier to understand, try reading the last line above as “for every number in the range 0-9, pick all with a value below 5 and make a list of them”. You can also embed such comprehensions directly in a function call like sum() without using [] around it.

In count_slots we have this code:

wield_usage = sum(
    getattr(slotobj, "size", 0)
    for slot, slotobj in slots.items()
    if slot is not WieldLocation.BACKPACK
)

We should be able to follow all except slots.items(). Since slots is a dict, we can use .items() to get a sequence of (key, value) pairs. We store these in slot and slotobj. So the above can be understood as “for every slot and slotobj-pair in slots, check which slot location it is. If it is not in the backpack, get its size and add it to the list. Sum over all these sizes”.

A less compact but maybe more readonable way to write this would be:

backpack_item_sizes = [] 
for slot, slotobj in slots.items(): 
    if slot is not WieldLocation.BACKPACK:
       size = getattr(slotobj, "size", 0) 
       backpack_item_sizes.append(size)
wield_usage = sum(backpack_item_sizes)

The same is done for the items actually in the BACKPACK slot. The total sizes are added together.

5.4.3. Validating slots

With these helpers in place, validate_slot_usage now becomes simple. We use max_slots to see how much we can carry. We then get how many slots we are already using (with count_slots) and see if our new obj’s size would be too much for us.

5.5. .add and .remove

We will make it so .add puts something in the BACKPACK location and remove drops it, wherever it is (even if it was in your hands).

# mygame/evadventure/equipment.py 

from .enums import WieldLocation, Ability

# ... 

class EquipmentHandler: 

    # ... 
     
    def add(self, obj):
        """
        Put something in the backpack.
        """
        self.validate_slot_usage(obj)
        self.slots[WieldLocation.BACKPACK].append(obj)
        self._save()

    def remove(self, slot):
        """
        Remove contents of a particular slot, for
        example `equipment.remove(WieldLocation.SHIELD_HAND)`
        """
        slots = self.slots
        ret = []
        if slot is WieldLocation.BACKPACK:
            # empty entire backpack! 
            ret.extend(slots[slot])
            slots[slot] = []
        else:
            ret.append(slots[slot])
            slots[slot] = None
        if ret:
            self._save()
        return ret

Both of these should be straight forward to follow. In .add, we make use of validate_slot_usage to double-check we can actually fit the thing, then we add the item to the backpack.

In .delete, we allow emptying by WieldLocation - we figure out what slot it is and return the item within (if any). If we gave BACKPACK as the slot, we empty the backpack and return all items.

Whenever we change the equipment loadout we must make sure to ._save() the result, or it will be lost after a server reload.

5.6. Moving things around

With the help of .remove() and .add() we can get things in and out of the BACKPACK equipment location. We also need to grab stuff from the backpack and wield or wear it. We add a .move method on the EquipmentHandler to do this:

# mygame/evadventure/equipment.py 

from .enums import WieldLocation, Ability

# ... 

class EquipmentHandler: 

    # ... 
    
    def move(self, obj): 
         """Move object from backpack to its intended `inventory_use_slot`.""" 
         
        # make sure to remove from equipment/backpack first, to avoid double-adding
        self.remove(obj) 
        
        slots = self.slots
        use_slot = getattr(obj, "inventory_use_slot", WieldLocation.BACKPACK)

        to_backpack = []
        if use_slot is WieldLocation.TWO_HANDS:
            # two-handed weapons can't co-exist with weapon/shield-hand used items
            to_backpack = [slots[WieldLocation.WEAPON_HAND], slots[WieldLocation.SHIELD_HAND]]
            slots[WieldLocation.WEAPON_HAND] = slots[WieldLocation.SHIELD_HAND] = None
            slots[use_slot] = obj
        elif use_slot in (WieldLocation.WEAPON_HAND, WieldLocation.SHIELD_HAND):
            # can't keep a two-handed weapon if adding a one-handed weapon or shield
            to_backpack = [slots[WieldLocation.TWO_HANDS]]
            slots[WieldLocation.TWO_HANDS] = None
            slots[use_slot] = obj
        elif use_slot is WieldLocation.BACKPACK:
            # it belongs in backpack, so goes back to it
            to_backpack = [obj]
        else:
            # for others (body, head), just replace whatever's there
            replaced = [obj]
            slots[use_slot] = obj
       
        for to_backpack_obj in to_backpack:
            # put stuff in backpack
            slots[use_slot].append(to_backpack_obj)
       
        # store new state
        self._save()

Here we remember that every EvAdventureObject has an inventory_use_slot property that tells us where it goes. So we just need to move the object to that slot, replacing whatever is in that place from before. Anything we replace goes back to the backpack.

5.7. Get everything

In order to visualize our inventory, we need some method to get everything we are carrying.

# mygame/evadventure/equipment.py 

from .enums import WieldLocation, Ability

# ... 

class EquipmentHandler: 

    # ... 

    def all(self):
        """
        Get all objects in inventory, regardless of location.
        """
        slots = self.slots
        lst = [
            (slots[WieldLocation.WEAPON_HAND], WieldLocation.WEAPON_HAND),
            (slots[WieldLocation.SHIELD_HAND], WieldLocation.SHIELD_HAND),
            (slots[WieldLocation.TWO_HANDS], WieldLocation.TWO_HANDS),
            (slots[WieldLocation.BODY], WieldLocation.BODY),
            (slots[WieldLocation.HEAD], WieldLocation.HEAD),
        ] + [(item, WieldLocation.BACKPACK) for item in slots[WieldLocation.BACKPACK]]
        return lst

Here we get all the equipment locations and add their contents together into a list of tuples [(item, WieldLocation), ...]. This is convenient for display.

5.8. Weapon and armor

It’s convenient to have the EquipmentHandler easily tell you what weapon is currently wielded and what armor level all worn equipment provides. Otherwise you’d need to figure out what item is in which wield-slot and to add up armor slots manually every time you need to know.

# mygame/evadventure/equipment.py 

from .objects import WeaponEmptyHand
from .enums import WieldLocation, Ability

# ... 

class EquipmentHandler: 

    # ... 
    
    @property
    def armor(self):
        slots = self.slots
        return sum(
            (
                # armor is listed using its defense, so we remove 10 from it
                # (11 is base no-armor value in Knave)
                getattr(slots[WieldLocation.BODY], "armor", 1),
                # shields and helmets are listed by their bonus to armor
                getattr(slots[WieldLocation.SHIELD_HAND], "armor", 0),
                getattr(slots[WieldLocation.HEAD], "armor", 0),
            )
        )

    @property
    def weapon(self):
        # first checks two-handed wield, then one-handed; the two
        # should never appear simultaneously anyhow (checked in `move` method).
        slots = self.slots
        weapon = slots[WieldLocation.TWO_HANDS]
        if not weapon:
            weapon = slots[WieldLocation.WEAPON_HAND]
        if not weapon:
            weapon = WeaponEmptyHand()
        return weapon

In the .armor() method we get the item (if any) out of each relevant wield-slot (body, shield, head), and grab their armor Attribute. We then sum() them all up.

In .weapon(), we simply check which of the possible weapon slots (weapon-hand or two-hands) have something in them. If not we fall back to the ‘fake’ weapon WeaponEmptyHand which is just a ‘dummy’ object that represents your bare hands with damage and all. (created in The Object tutorial earlier).

5.9. Extra credits

This covers the basic functionality of the equipment handler. There are other useful methods that can be added:

  • Given an item, figure out which equipment slot it is currently in

  • Make a string representing the current loadout

  • Get everything in the backpack (only)

  • Get all wieldable items (weapons, shields) from backpack

  • Get all usable items (items with a use-location of BACKPACK) from the backpack

Experiment with adding those. A full example is found in evennia/contrib/tutorials/evadventure/equipment.py.

5.10. Unit Testing

Create a new module mygame/evadventure/tests/test_equipment.py.

To test the EquipmentHandler, easiest is create an EvAdventureCharacter (this should by now have EquipmentHandler available on itself as .equipment) and a few test objects; then test passing these into the handler’s methods.

# mygame/evadventure/tests/test_equipment.py 

from evennia.utils import create 
from evennia.utils.test_resources import BaseEvenniaTest 

from ..objects import EvAdventureRoom
from ..enums import WieldLocation

class TestEquipment(BaseEvenniaTest): 
    
    def setUp(self): 
        self.character = create.create_object(EvAdventureCharacter, key='testchar')
        self.helmet = create.create_object(EvAdventureHelmet, key="helmet") 
        self.weapon = create.create_object(EvAdventureWeapon, key="weapon") 
         
    def test_add_remove): 
        self.character.equipment.add(self.helmet)
        self.assertEqual(
            self.character.equipment.slots[WieldLocation.BACKPACK],
            [self.helmet]
        )
        self.character.equipment.remove(self.helmet)
        self.assertEqual(self.character.equipment.slots[WieldLocation.BACKPACK], []) 
        
    # ...

5.11. Summary

Handlers are useful for grouping functionality together. Now that we spent our time making the EquipmentHandler, we shouldn’t need to worry about item-slots anymore - the handler ‘handles’ all the details for us. As long as we call its methods, the details can be forgotten about.

We also learned to use hooks to tie Knave’s custom equipment handling into Evennia.

With Characters, Objects and now Equipment in place, we should be able to move on to character generation - where players get to make their own character!