About

tcod-ecs is a Sparse-set Entity-component-system implemented using Python's dict and set types. See the ECS FAQ for more info.

This implementation focuses on type-hinting, organization, and is designed to work well with Python. The following features are currently implemented:

• Entities can store components which are instances of any Python object. Components are looked up by their type.
• Entities can have one instance of a type, or multiple instances of a type using a hashable tag to differentiate them.
• Entity relationships are supported, either as many-to-many or many-to-one relationships.
• ECS Queries can be made to fetch entities having a combination of components/tags/relations or excluding such.
• The ECS Registry object can be serialized with Python's pickle module for easy storage.

A lightweight version which implements only the entity-component framework exists called tcod-ec. tcod-ec was geared towards a dynamic-typed-dict style of syntax and is missing a lot of important features such as queries and named components.

Installation

Use pip to install this library:

pip install tcod-ecs

If tcod is installed and the version is less than 14.0.0 then import tcod.ecs will fail. Remove or update tcod to fix this issue.

Examples

Registry

The ECS Registry is used to create and store entities and their components.

>>> import tcod.ecs
>>> registry = tcod.ecs.Registry()  # New empty registry

Entity

Each Entity is identified by its unique id (uid) which can be any hashable object combined with the registry it belongs. New unique entities can be created with Registry.new_entity which uses a new object() as the uid, this guarantees uniqueness which is not always desireable. An entity always knows about its assigned registry, which can be accessed with the Entity.registry property from any Entity instance. Registries only know about their entities once the entity is assigned a name, component, tag, or relation.

>>> entity = registry.new_entity()  # Creates a unique entity using `object()` as the uid
>>> entity
<Entity(uid=object at ...)>
>>> entity.registry is registry  # Registry can always be accessed from their entity
True
>>> registry[entity.uid] is entity  # Entities with the same registry/uid are compared using `is`
True

# Reference an entity with the given uid, can be any hashable object:
>>> entity = registry["MyEntity"]
>>> entity
<Entity(uid='MyEntity')>
>>> registry["MyEntity"] is entity  # Matching entities ALWAYS share a single identity
True

Use Registry.new_entity to create unique entities and use Registry[x] to reference a global entity or relation with an id. registry[None] is recommend for use as a global entity when you want to store components in the registry itself.

Do not save the uid's of entities to be used later with registry[uid], this process is slower than holding onto the Entity instance.

Serialization

Registries are normal Python objects and can be pickled as long as all stored components are pickleable.

>>> import pickle
>>> pickled_data: bytes = pickle.dumps(registry)
>>> registry = pickle.loads(pickled_data)

Stability is a priority but changes may still break older saves. Backwards compatibility is not a priority, pickled registries should not be unpickled with an older version of the library. This project follows Semantic Versioning, major version increments will break the API, the save format or both, minor version increments may break backwards compatibility. Check the changelog to be aware of format changes and breaks. There should always be a transition period before a format break, so keeping up with the latest version is a good idea.

Components

Components are instances of any Python type. These can be accessed, assigned, or removed from entities via the dict-like Entity.components attribute. The type is used as the key to access the component. The types used can be custom classes or standard Python types.

>>> import attrs
>>> entity = registry.new_entity()
>>> entity.components[int] = 42
>>> entity.components[int]
42
>>> int in entity.components
True
>>> del entity.components[int]
>>> entity.components[int]  # Missing keys raise KeyError
Traceback (most recent call last):
  ...
KeyError: <class 'int'>
>>> entity.components.get(int, "default")  # Test keys with `.get()` like a dictionary
'default'
>>> @attrs.define
... class Vector2:
...     x: int = 0
...     y: int = 0
>>> entity.components[Vector2] = Vector2(1, 2)
>>> entity.components[Vector2]
Vector2(x=1, y=2)
>>> entity.components |= {int: 11, Vector2: Vector2(0, 0)}  # Multiple values can be assigned like a dict
>>> entity.components[int]
11
>>> entity.components[Vector2]
Vector2(x=0, y=0)

# Queries can be made on all entities of a registry with matching components
>>> for e in registry.Q.all_of(components=[Vector2]):
...     e.components[Vector2].x += 10
>>> entity.components[Vector2]
Vector2(x=10, y=0)

# You can match components and iterate over them at the same time.  This can be combined with the above
>>> for pos, i in registry.Q[Vector2, int]:
...     print((pos, i))
(Vector2(x=10, y=0), 11)

# You can include `Entity` to iterate over entities with their components
# This always iterates over the entity itself instead of an Entity component
>>> for e, pos, i in registry.Q[tcod.ecs.Entity, Vector2, int]:
...     print((e, pos, i))
(<Entity...>, Vector2(x=10, y=0), 11)

Named Components

Only one component can be assigned unless that component is given a unique name. You can name components with the key syntax (name, type) when assigning components. Names are not limited to strings, they are a tag equivalent and can be any hashable or frozen object. The syntax [type] and [(name, type)] can be used interchangeably in all places accepting a component key. Queries on components access named components with the same syntax and must use names explicitly.

>>> entity = registry.new_entity()
>>> entity.components[Vector2] = Vector2(0, 0)
>>> entity.components[("velocity", Vector2)] = Vector2(1, 1)
>>> entity.components[("velocity", Vector2)]
Vector2(x=1, y=1)
>>> @attrs.define(frozen=True)
... class Slot:
...     index: int
>>> entity.components |= {  # Like a dict Entity.components can use |= to update items in-place
...     ("hp", int): 10,
...     ("max_hp", int): 12,
...     ("atk", int): 1,
...     str: "foo",
...     (Slot(1), str): "empty",
... }
>>> entity.components[("hp", int)]
10
>>> entity.components[str]
'foo'
>>> entity.components[(Slot(1), str)]
'empty'

# Queries can be made on all named components with the same syntax as normal ones
>>> for e in registry.Q.all_of(components=[("hp", int), ("max_hp", int)]):
...     e.components[("hp", int)] = e.components[("max_hp", int)]
>>> entity.components[("hp", int)]
12
>>> for e, pos, delta in registry.Q[tcod.ecs.Entity, Vector2, ("velocity", Vector2)]:
...     e.components[Vector2] = Vector2(pos.x + delta.x, pos.y + delta.y)
>>> entity.components[Vector2]
Vector2(x=1, y=1)

Tags

Tags are hashable objects stored in the set-like Entity.tags. These are useful as flags or to group entities together.

>>> entity = registry.new_entity()
>>> entity.tags.add("player")  # Works well for groups
>>> "player" in entity.tags
True
>>> entity.tags.add(("eats", "fruit"))
>>> entity.tags.add(("eats", "meat"))
>>> set(registry.Q.all_of(tags=["player"])) == {entity}
True

Relations

Entity relations are unidirectional from an origin entity to possibly multiple target entities.

• Use origin.relation_tag[tag] = target to associate an origins tag exclusively with the target entity. This uses standard assignment and is useful for tags which would not make sense with multiple targets. Reading origin.relation_tag[tag] returns a single target while enforcing the invariant of only having one target.
• Use origin.relation_tags_many[tag].add(target) to associate a tag with multiple targets. This supports set-like syntax such as adding or removing multiple targets at once. This allows for many-to-many relations.
• Use origin.relation_components[component_key][target] = component to associate a target entity with a component. This allows storing data along with a relation. This supports dict-like syntax. The component_key can be queried like a normal tag.

Relation queries

Relations are queried with registry.Q.all_of(relations=[...]). This expects 2-item or 3-item tuples following these rules:

• Use (tag, target) to match the origin entities with the relation tag to target.
• If tag is a component key then component relations are also matched. This means you should be careful with tags which look like component keys.
• target can be a specific entity. This means only entities relating to that specific entity will be matched.
• target can be query itself. This means only entities relating to a match from the sub-query are matched.
• target can be ... which means an entity with a relation to any entity is matched.
• To reverse the direction use a 3-item tuple (origin, tag, None). origin can be anything a target could be.

Relations using sub-queries may be chained together. See Sander Mertens - Why it is time to start thinking of games as databases to understand the repercussion of this.

You can use the following table to help with constructing relation queries:

Matches	Syntax
Origins with a relation tag to target_entity	(tag, target_entity)
Origins with a relation tag to any target entity	(tag, ...) (Literal dot-dot-dot)
Origins with a relation tag to any targets matching a sub-query	(tag, registry.Q.all_of(...))
Targets of the relation tag from origin_entity	(origin_entity, tag, None)
Targets of the relation tag from any origin entity	(..., tag, None) (Literal dot-dot-dot)
Targets of the relation tag from any origins matching a sub-query	(registry.Q.all_of(...), tag, None)

>>> @attrs.define
... class OrbitOf:  # OrbitOf component
...     dist: int
>>> LandedOn = "LandedOn"  # LandedOn tag
>>> star = registry.new_entity()
>>> planet = registry.new_entity()
>>> moon = registry.new_entity()
>>> ship = registry.new_entity()
>>> player = registry.new_entity()
>>> moon_rock = registry.new_entity()
>>> planet.relation_components[OrbitOf][star] = OrbitOf(dist=1000)
>>> moon.relation_components[OrbitOf][planet] = OrbitOf(dist=10)
>>> ship.relation_tag[LandedOn] = moon
>>> moon_rock.relation_tag[LandedOn] = moon
>>> player.relation_tag[LandedOn] = moon_rock
>>> set(registry.Q.all_of(relations=[(OrbitOf, planet)])) == {moon}
True
>>> set(registry.Q.all_of(relations=[(OrbitOf, ...)])) == {planet, moon}  # Get objects in an orbit
True
>>> set(registry.Q.all_of(relations=[(..., OrbitOf, None)])) == {star, planet}  # Get objects being orbited
True
>>> set(registry.Q.all_of(relations=[(LandedOn, ...)])) == {ship, moon_rock, player}
True
>>> set(registry.Q.all_of(relations=[(LandedOn, ...)]).none_of(relations=[(LandedOn, moon)])) == {player}
True