PECS

PECS is the ✨Respectably Muscled✨ Python ECS library that aims to provide a powerful, user-friendly, and fast-as-hell framework for game development.

This library is the spiritual successor to my prior ECS library, ECStremity. Both this and its predecessor were inspired by the JavaScript ECS library geotic, created and maintained by @ddmills. I highly recommend checking out that project as well as the excellent resources cited in its README.

What is ECS, you ask? Check it out!

Installation

Install the package from PyPI using pip:

pip install pecs-framework

Or grab it directly from this repository:

pip install git+https://github.com/krummja/PECS

Usage and Examples

To start flexing your PECS, import the library and set up some components. Components can be built as standard Python classes:

import pecs_framework as pecs


class Position(pecs.Component):
    """Representation of an Entity's position in 2D space."""

    def __init__(self, x: int = 0, y: int = 0) -> None:
        self.x = x
        self.y = y

    @property
    def xy(self) -> tuple[int, int]:
        return self.x, self.y

As extensions of existing components:

import pecs_framework as pecs


class Velocity(Position):
    """Representation of an Entity's velocity in 2D space."""

Or as dataclasses:

import pecs_framework as pecs
from dataclasses import dataclass, field


@dataclass
class Health(pecs.Component):
    """Representation of an Entity's health."""
    maximum: int = 100
    current: int = field(init=False)

    def __post_init__(self) -> None:
        self.current = self.maximum

Components can have as much or as little behavior as needed, although it is generally better to keep to a strict single-repsonsibility principle. We can even have components that have no behavior at all, representing boolean flags for queries:

import pecs_framework as pecs


class IsFrozen(pecs.Component):
    """Flag component denoting a frozen entity."""

Queries

The easiest way to build out systems is through world queries. To make a system that tracks and updates the components relevant to movement, we might query for Position and Velocity components. Because we want our entities to move, we want to exclude those marked with the IsFrozen flag. Perhaps we also want to grab only those entities that can fly through Wings or swim through Fins:

import pecs_framework as pecs


ecs = pecs.Engine()
domain = ecs.create_domain("World")

kinematics = domain.create_query(
    all_of = [
        Position, 
        Velocity
    ],
    any_of = [
        Wings, 
        Fins
    ],
    none_of = [
        IsFrozen
    ],
)

Queries can specify all_of, any_of, or none_of quantifiers. The query in the example above asks for entities that must have both Position and Velocity, may have (inclusive) Wings or Fins, and must not have IsFrozen.

We can access the result set of the query and do some operation on them every loop cycle:

def process(dt):
    for entity in targets.result:
        entity[Position].x += entity[Velocity].x * dt
        entity[Position].y += entity[Velocity].y * dt

For convenience, the library provides barebones system class that you can extend for your own purposes:

import pecs_framework as pecs


class MovementSystem(pecs.BaseSystem):

    def initialize(self) -> None:
        self.query(
            'movable',
            all_of = [Position, Velocity],
            none_of = [IsFrozen],
        )

    def update(self) -> None:
        movables = self._queries
        for entity in movables:
            entity[Position].x += entity[Velocity].x
            entity[Position].y += entity[Velocity].y

---

Warning:

Do not override the __init__ method of BaseSystem -- use the provided initialize method instead.

---

Broadcasting Events to Components

Complex interactions within and among entities can be achieved by firing events on an entity. This creates an EntityEvent that looks for methods on all of the entity's methods prefixed with on_.

zombie.fire_event('attack', {
    'target': survivor,
    'multiplier': 1.5,
})

On the zombie entity, we might have attached an Attacker component with the following logic:

class Attacker(pecs.Component):

    def __init__(self, strength: int) -> None:
        self.strength = strength

    def on_attack(self, evt: pecs.EntityEvent) -> pecs.EntityEvent:
        target: Entity = evt.data.target
        target.fire_event('damage_taken', {
            'amount': self.strength * evt.data.pultiplier,
        })
        evt.handle()
        return evt

When we execute fire_event with the event name attack, the event system will find all on_attack methods on that entity's components. If we want the event propagation to stop at a particular component, we can call evt.handle() which will immediately break broadcasting down the component list. This means that we can potentially have any number of components respond to a specific event, although it may generally be safer to fire a secondary event to prevent ordering issues.

Internally, the EntityEvent class puts together an instance of the class EventData, which provides access to the properties defined in the fire_event call.

zombie.fire_event('attack', {
    'target': survivor,                 # <-- We defined 'target' here
    'multiplier': 1.5                   # <-- and 'multiplier' here
})

def on_attack(self, evt: pecs.EntityEvent) -> pecs.EntityEvent:
    target = evt.data.target            # --> survivor
    multiplier = evt.data.multiplier    # --> 1.5

Actions can also be defined as a tuple and passed into the fire_event method. This allows for easy abstraction over variables used in the event:

attack_against = (lambda target : ('attack', {
    'target': target,
    'multiplier': 1.5
}))

zombie.fire_event(attack_against(survivor))

Creating Entities from Prefabs

PECS supports defining prefab entities with preconfigured component properties. Define prefabs as .json files and register them with the engine:

{
  "name": "GameObject",
  "inherit": [],
  "components": [
    {
      "type": "Position"
    },
    {
      "type": "Renderable",
      "properties": {
        "ch": "?",
        "bg": [0, 0, 0],
      }
    },
    {
      "type": "Noun"
    }
  ]
}

import pecs_framework as pecs
import os


ROOTDIR = os.path.dirname(__file__)
PREFABS = os.path.join(ROOTDIR, 'prefabs')


ecs = pecs.Engine()
ecs.prefabs.register(PREFABS, 'game_object')

Now PECS will look for a file named game_object.json in the specified prefabs path and automatically load it for you. We can build an entity using this prefab very easily:

game_object = ecs.domain.entities.create_from_prefab(
    template = 'GameObject',
    properties = {
        'Position': {
            'x': 15,
            'y': 10,
        },
        'Renderable': {
            'fg': [255, 0, 255],
        },
        'Noun': {
            'text': 'Test Object'
        }
    },
    alias = 'test_object_01',
)

Prefabs can specify other prefabs to inherit from as well. Prefabs can be defined as hierarchies of any depth and breadth. Note that properties will always be resolved from the most deeply embedded prefab to the least, overwriting with the most recent specification. If no properties are passed in the prefab or when creating from prefab, defaults from the component itself will be used.

For examples, check out the tests folder in this repository.