⚠️ Beta State

PyBevy is in an early and experimental stage. The API is incomplete, subject to breaking changes without notice, and you should expect bugs. Many features are still under development.

Hierarchy

Create parent-child entity relationships with automatic transform propagation.

Introduction

Entity hierarchies let child entities inherit their parent's transform. When the parent moves or rotates, all children follow automatically. This is essential for articulated objects, UI layouts, and scene graphs.

from pybevy.prelude import *

Setup

Create a parent cube with a child cube attached. When the parent rotates, the child orbits around it.

@component
class Rotator(Component):
    pass
 
 
def setup(
    commands: Commands,
    meshes: ResMut[Assets[Mesh]],
    materials: ResMut[Assets[StandardMaterial]],
) -> None:
    cube = meshes.add(Cuboid(2.0, 2.0, 2.0))
    material = materials.add(Color.srgb(0.8, 0.7, 0.6))
 
    commands.spawn(
        Mesh3d(cube), MeshMaterial3d(material), Transform.from_xyz(0.0, 0.0, 1.0), Rotator()
    ).with_children(
        lambda parent: parent.spawn(Mesh3d(cube), MeshMaterial3d(material), Transform.from_xyz(0.0, 0.0, 3.0))
    )
 
    commands.spawn(PointLight(shadows_enabled=True), Transform.from_xyz(4.0, 8.0, 4.0))
    commands.spawn(Camera3d(), Transform.from_xyz(0.0, 5.0, 10.0).looking_at(Vec3.ZERO, Vec3.Y))

Rotation System

Rotate only entities with the Rotator marker. The child inherits this rotation and orbits the parent.

def rotate(time: Res[Time], query: Query[Mut[Transform], With[Rotator]]) -> None:
    for transform in query:
        transform.rotate_x(3.0 * time.delta_secs())

Running the App

@entrypoint
def main(app: App) -> App:
    return (
        app
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, rotate)
    )
 
if __name__ == "__main__":
    main().run()

Running this example

Use PyBevy's hot reload feature to run and develop this example. If you don't have PyBevy installed, check out the Quick Start guide.

$pybevy watch hierarchy.py

The code will reload automatically when you make changes to the file.

From Python to Rust

Notice how the core concepts in the code—Commands, Assets, App, and Systems—are identical to the original Bevy example?

This is the power of pybevy! It lets you learn Bevy's powerful, data-driven architecture in friendly Python.

When your project grows and you're ready for maximum, native performance, you'll already know the concepts to start writing systems in Bevy Engine with Rust.