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.
Fixed Timestep
Compare Update (every frame) with FixedUpdate (fixed time intervals).
Introduction
Update systems run every frame (variable rate). FixedUpdate systems run at a fixed interval (default 64Hz), regardless of frame rate. Fixed timestep is essential for deterministic physics.
from pybevy.prelude import *Frame Counter
_update_count = [0]
_fixed_count = [0]Update System
Runs every rendered frame — the interval depends on your display refresh rate and rendering load.
def update_system(time: Res[Time]) -> None:
_update_count[0] += 1
if _update_count[0] % 60 == 0:
print(f"Update #{_update_count[0]} at {time.elapsed_secs():.2f}s")Fixed Update System
Runs at a fixed interval. The timestep is always the same, making it ideal for physics simulation.
def fixed_update_system(time: Res[Time]) -> None:
_fixed_count[0] += 1
if _fixed_count[0] % 64 == 0:
print(f"FixedUpdate #{_fixed_count[0]} at {time.elapsed_secs():.2f}s")Running the App
@entrypoint
def main(app: App) -> App:
return (
app
.add_plugins(DefaultPlugins)
.add_systems(Update, update_system)
.add_systems(FixedUpdate, fixed_update_system)
)
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.
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.