microecs 0.3.1

MicroECS: Minimal Entity Component System (ECS) in python and numpy

MicroECS

Minimal (~300 LoC) Entity Component System in python and numpy. Examples also use raylib for rendering.

Usage:

• pip install -r requirements.txt
• Sandbox: python main.py
• Tests: pytest test/

There are only 4 primitives (bottom up): Component, Pool, QueryResult, World:

• Component is a simple python dataclass holding only data. All entries must be numpy arrays with metadata fields: shape and dtype. We support 5 dtypes only: int32, float32, bool, str and object. A component with no fields is a valid tag for querying (e.g. class Frozen(Component): pass).
• Pool is a simple 'archetype' dynamic array, holding entities of the same type (same set of components). Usses Components metadata to construct contiguous arrays for all entities of the same type.
• QueryResult is a list of pools that match some query on all the entities of the World. It acts as a contiguous numpy-like container that implements numpy's __array_function__ and __array_ufunc__. For all intents and purposes it should feel like a (N, ...) view over all the entities. If you need a numpy array (not all ops are supported, for e.g. indexing on the first axis), use QueryResult.numpy() (see the field numpy contract below). It also exposes entity_ids: a flat (N,) array of the matched entities' ids, in the same pool-by-pool order as the fields, so you can zip(qr.entity_ids, qr.position) or feed an id back to world.get_entity / world.remove_entity.
• World is a manager of Pools and has an overview of all the entities in the scene. It also manages the migration of entities from one pool to the other. A World can also require extra metadata keys on every field via World(extra_metadata=["serializable"]), to enforce component-level behavior such as field serialization.

Few relevant concepts:

• Pool operates on array indices, while World operates on entity IDs (also integers). This allows seamless movement between pools while the high-level systems still working as intended.
• All mutable operations on World are lazy. These are: add_entity, remove_entity, add_component, remove_component. They are added to a command buffer which is only executed when calling world.update().
• Systems are a convention, they are not part of this library. They can be defined at application level and act as hooks or callbacks. The World object doesn't need to know more than entities and components.

Super simplified main loop structure:

from typing import Callable
import numpy as np
import raylib as rl
from microecs import World, Component

# components
class HasPosition(Component):
    # 'shape' + 'dtype' are always required. For additional metadata (e.g. examples/03-serialization) use extra_metadata
    position: np.ndarray = field(metadata={"shape": (2, ), "dtype": "float32"})
class HasVelocity(Component):
    velocity: np.ndarray = field(metadata={"shape": (2, ), "dtype": "float32"})
class HasColor(Component):
    color: np.ndarray = field(metadata={"shape": (4, ), "dtype": "int32"})

# systems: Note they are a convention!
class RenderSystem:
    def __call__(self, world: World):
        query_result = world.query(HasPosition, HasColor, exclude=[]) # contiguous-like view of all entities matching
        for position, color in zip(query_result.position, query_result.color): # draw each entity
            DrawEntity(position, color)

class MotionSystem:
    def __call__(self, world: World):
        qr = world.query(HasPosition, HasVelocity) # 'exclude' is optional
        qr.position[:] = qr.position + qr.velocity * DT # writes back to all the underlying pools using numpy's rules
        # Alternative for per-pool update. Less ergonomic, but maybe faster in extreme cases as it avoids the _Field obj
        for pool in qr.pool_list:
            pool.position[:] = pool.position + pool.velocity * DT

def main():
    render_system: list[Callable] = RenderSystem()
    update_systems: list[Callable] = [MotionSystem()]

    world = World(components=[HasPosition, HasColor, HasVelocity], extra_metadata=None) # extra_metadata is optional
    for _ in range(n_objects):
        # NOTE: world.{add/remove}_{entity/component} are lazy. They take effect after the first world.update() call.
        world.add_entity(components=(HasPosition, HasVelocity, HasColor), # tuple of components (types)
                         position=           np.array((x, y), "float32"), # data as kwargs
                         color=         np.array("black", dtype="int32"),
                         velocity=         np.array((vx, vy), "float32"))

    while not rl.WindowShouldClose():
        world.update() # must be called at each tick so the lazy methods are processed and entities are updated
        # update stuff...
        _ = [system(world=world) for system in update_systems]
        # draw stuff, e.g. using raylib
        rl.BeginDrawing()
        rl.ClearBackground(rl.RAYWHITE)
        rl.DrawFPS(rl.GetScreenWidth() - 100, 0)
        render_system(world=world)
        rl.EndDrawing()

The field (_Field) numpy contract

qr.position returns a _Field: a view over the matching pools that behaves like one (N, *e) numpy array (e.g. (N, 2) for a (2,) field). It applies each op per pool and stitches the result back. So the contract is exactly:

Any op that treats rows independently behaves identically to numpy on the concatenated (N, *e) array — same values, same shape, and the same error on bad shapes.

That covers elementwise math and ufuncs, broadcasting (every operand shape numpy accepts, and it raises on the ones numpy rejects), comparisons, np.where / np.clip, batched np.matmul, feature-axis reductions/indexing (np.linalg.norm(qr.velocity, axis=1), qr.pose[:, 0]), and write-back broadcasting (qr.position[:] = <scalar | (*e,) | (N, *e)>). Pinned in test/unit/test_field_numpy_parity.py.

Edge cases worth knowing:

• Not a full ndarray — these raise, never lie. Entity-axis indexing beyond a single qr.f[i] (qr.f[:], qr.f[2:4], qr.f[mask], fancy), partial entity writes, and ndarray methods/attrs (.sum(), .mean(), .dtype, .ndim, .T). Need any of these? Materialize first with qr.f.numpy().
• Axis-0 ops are per-pool, not global (footgun). np.sort / np.cumsum / np.sum over axis=0 run within each pool and reset at pool boundaries — they do not see all entities at once, so they differ from numpy. They're allowed, but if you want a global result, do qr.f.numpy() first. A reduction that collapses the entity axis is rejected when its length no longer matches the pool's row count.
• Operands must come from the same query. Alignment is per-pool, not by flat index, so don't mix a _Field from one world.query(...) into an op on another.