KayPy

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A real-Python game engine with KAPLAY's API — runs on your machine, exports to the browser.

Every name is KAPLAY's own name, in KAPLAY's own order, with KAPLAY's own arguments: add, sprite, onKeyDown, loadSpriteAtlas, isGrounded. So KAPLAY's documentation and every KAPLAY example on the internet still tells you what to write — you just write it in Python.

Nothing is transpiled and nothing runs through a JavaScript bridge. It's pygame-ce underneath, all the way down. The same file runs as a desktop window with python game.py, or as a web page with kaypy web game.py.

Get running

pip install kaypy
kaypy new mygame
cd mygame
python game.py

Four commands and you have a game on screen — kaypy new writes a working game.py along with the sprites to build on, so there is nothing to hunt for and no paths to fix. Arrow keys to move, space to jump. Then open game.py and start changing it.

It also fetches the three lesson sounds, which are 2.6 MB and live outside the package so that every install doesn't pay for audio one lesson plays. If that fetch fails — no internet, a school network that blocks GitHub — you get a sentence saying so and a game that still runs, because the starter game plays no sound. kaypy sounds mygame picks them up whenever you like.

Put the same file on the web with:

kaypy web game.py

which builds it into one HTML file. Double-click that file and the game plays — no server, nothing installed, nothing unzipped. Upload the same single file to itch.io, email it, or drop it on a school share.

New here? The guide is a thirteen-lesson course that starts from nothing and ends with a state-machine enemy AI — the Learn Kaplay lessons, written for Python. It is also in this repository as GUIDE.md.

Nothing to install: the playground runs kaypy in the browser, with the sprites already there. Write a game in a tab and download it as a single HTML file.

Stuck, or teaching with this? There is a Discord.

from kaypy import *

kaypy(width=800, height=600, background=[141, 183, 255])

loadSprite("bean", "images/bean.png")

add([
    sprite("bean"),
    pos(120, 80),
    area(),
])

That's a complete, running game. Save it, run python game.py, and a window opens. There is no run() call — exactly like the browser version, the loop is already going the moment kaypy() executes.

Quick overview

Game objects are made of components. You don't subclass anything; you hand add() a list of parts, and the object is whatever those parts make it.

player = add([
    sprite("bean"),
    pos(100, 200),
    area(),           # a collision box — without this, nothing can hit it
    body(),           # gravity, jumping, isGrounded()
    anchor("bot"),    # pos() now means "where its feet are"
    "player",         # a plain string is a tag
])

Each component adds its own methods to the object. body() is what gives you .jump() and .isGrounded(); ask for them without it and you get a plain AttributeError telling you so.

Input is callbacks — put the event above the function it runs.

SPEED = 320

@onKeyDown("left")                       # every frame while held
def go_left():
    player.move(-SPEED, 0)


@onKeyPress("space")                     # once, when the key goes down
def jump():
    if player.isGrounded():
        player.jump(1000)

Every event also takes a function directly, which is shorter when the handler is one short line and nothing else:

onKeyDown("right", lambda: player.move(SPEED, 0))

Both forms do exactly the same thing, so use whichever reads better. The decorator is the one to reach for first, because a lambda can only hold a single expression and the workarounds get ugly fast: as a lambda, the jump above has to be written lambda: player.jump(1000) if player.isGrounded() else None — where the else None does nothing at all and is only there to satisfy Python — and a handler that assigns something has to reach for lambda: setattr(player, "pos", spawn), because a lambda cannot contain =. As a decorator both are just ordinary indented code. Named handlers also show up in error messages as jump rather than <lambda>.

Collisions are events, addressed by tag.

@player.onCollide("danger")
def died(spike):
    player.pos = level.tile2Pos(2, 5)
    go("gameover")


player.onCollide("coin", lambda coin: coin.destroy())

Only objects with area() collide at all. An object with area() and body() is solid; one with area() alone is a trigger you pass straight through — that's how a coin can be picked up while a wall stops you.

Turning, and going the way you point. rotate() gives an object an angle; Vec2.fromAngle() turns that angle back into a direction, which is what "thrust forward" means once a thing can face anywhere.

ship = add([sprite("ship"), pos(center()), anchor("center"), rotate(0)])
ship.vel = vec2(0, 0)


@onKeyDown("left")
def turn_left():
    ship.rotateBy(-200 * dt())


@onKeyDown("right")
def turn_right():
    ship.rotateBy(200 * dt())


@onKeyDown("up")
def thrust():
    ship.vel = ship.vel + Vec2.fromAngle(ship.angle) * 320 * dt()

That last one is the case the decorator exists for: thrust assigns, and a lambda cannot contain =, so as a one-liner it would have to be lambda: setattr(ship, "vel", ...).

Rotation is about the anchor, so anchor("center") spins on the spot and the default top-left corner swings around it. The collision box does not turn — it stays the upright rectangle, which is what Kaplay does and what keeps a spinning asteroid's hitbox from growing and shrinking as it goes round.

examples/asteroids.py is the whole thing: turning, thrust, momentum, screen-wrap and splitting rocks.

The mouse does more than click. onClick tells you that someone clicked. For aiming, dragging or holding, you need to know which button and whether it is down right now.

@onMousePress("left")
def shoot():
    heading = (mousePos() - player.pos).unit()
    add([sprite("bullet"), pos(player.pos), move(heading, 500), lifespan(2)])

@onUpdate
def charge():
    if isMouseDown("left"):          # held, not just pressed
        power = min(power + dt(), 1)

Drawing that is not a game object. A health bar, an aim line, a grid — a mark on the screen for one frame, not a thing in the world. Those go in onDraw, and the draw functions only work there.

@onDraw
def hud():
    # fixed=True ignores the camera, which is what a HUD wants
    drawRect(pos=vec2(20, 20), width=200, height=16, color=rgb(60, 0, 0),
             fixed=True)
    drawRect(pos=vec2(20, 20), width=20 * player.hp, height=16,
             color=rgb(220, 40, 40), fixed=True)
    drawLine(p1=player.pos, p2=mousePos(), width=2, color=rgb(255, 255, 0))

Hit points, and something to do when they run out.

enemy = add([sprite("ogre"), pos(300, 200), area(), health(3), "enemy"])

@enemy.onDeath
def slain():
    addKaboom(enemy.pos)
    enemy.destroy()

enemy.hp = 3 does the same until the third place that takes a point off, because now three lines have to remember to check for zero — and the one that forgets is the enemy that cannot be killed. hurt() is one place, and death is one event that fires exactly once however many things land in the same frame.

A high score that is still there tomorrow.

best = getData("best_score", 0)

@player.onCollide("spike")
def died():
    if score > best:
        setData("best_score", score)
    go("gameover", score)

On your machine that is a kaypy-data.json file beside the game, which you can open and read — and delete, to start over. In a browser it is the page's own storage. If saving is not possible at all, the game says so once and carries on without it.

Gravity needs a world to fall in.

setGravity(2400)

add([                     # the floor
    rect(width(), 40),
    pos(0, 560),
    area(),
    body(isStatic=True),  # solid, and nothing can push it
])

Moving something smoothly is a tween. You say where it starts, where it ends, how long it takes, and what to do with each value along the way.

box = add([rect(60, 60), pos(100, 300), opacity(1)])


def move_box(x):
    box.pos.x = x


tween(100, 600, 0.5, move_box, easings.easeOutBounce)
tween(1.0, 0.0, 1.0, lambda a: setattr(box, "opacity", a)).then(
    lambda: box.destroy())

Numbers, vec2 positions and colour tuples all tween. The fifth argument is the easing — the shape of the motion — and without one everything travels at a flat, robotic pace. There are thirty-one, named easeIn, easeOut and easeInOut for each of Sine, Quad, Cubic, Quart, Quint, Expo, Circ, Back, Elastic and Bounce, plus easings.linear for none at all. Any function from 0–1 to 0–1 works too.

Unlike the events, tween is not a decorator: the function it takes is a setter that receives every value along the way, not a handler that runs once.

Whole levels are drawn as pictures made of characters.

level = addLevel([
    "  $    $  ",
    "          ",
    "@   ^^    ",
    "==========",
], {
    "tileWidth": 64,
    "tileHeight": 64,
    "tiles": {
        "=": lambda: [sprite("grass"), area(), body(isStatic=True)],
        "$": lambda: [sprite("coin"), area(), "coin"],
        "^": lambda: [sprite("spike"), area(), "danger"],
        "@": lambda: [sprite("bean"), area(), body(), anchor("bot"), "player"],
    },
})

player = level.get("player")[0]

Those lambdas are not event handlers — each one is a recipe that gets called every time that character appears, to build a fresh list of components. A lambda returning a list is exactly the right shape for that, so they stay lambdas.

Scenes are named screens you jump between.

@scene("game")
def build_game():
    build_the_level()


@scene("gameover")
def show_gameover(score):
    show_score(score)


go("game")                 # start here
# ...later, from anywhere:
go("gameover", player_score)

Installing

pip install kaypy

Python 3.10 or newer, and pygame-ce is the only dependency. There is nothing else to install for the web export — it used to need pygbag and an ffmpeg to convert sounds, and needs neither now.

pip install kaypy gives you two things: the kaypy package to import, and a kaypy command with three subcommands.

Working from a clone instead? pip install -e . from the project root, and use python webbuild.py game.py wherever this README says kaypy web game.py — they run the same code.

Putting a game on the web

kaypy web game.py          # or: python webbuild.py game.py, from a clone

One command, one file:

Built game.py as one file:
    web_build/game.html
    412 KB, including 3 assets

Double-click it and it plays. It reads your script to find the images and sounds it loads and carries those, and only those, inside the file — along with the engine and your program. There is nothing beside it to keep together, nothing to zip, and no server to start.

To publish on itch.io: upload that one file, set Kind of project to HTML, and tick This file will be played in the browser. Their own instructions cover it — "For simple projects that are self contained in a single .html file, you directly upload the file without zipping it."

Your program goes in unchanged. kaypy opens sprites as ordinary files, and the page gives it a filesystem to open them from, so loadSprite("bean", "images/bean.png") means the same thing in a built game as it does on your desktop. Open the built file in a text editor and your own code is in there, as you wrote it.

The one thing it fetches is Python itself — Pyodide and pygame-ce, from a CDN, on the first run. So a built game wants an internet connection the first time it is opened and takes a few seconds to start; the browser caches both afterwards. Embedding them would make every game tens of megabytes.

Building needs no internet at all — it is your own machine, your own files, and the engine already installed. Only playing a built game fetches anything, and only Python itself, once.

Your game file doesn't change between the two targets — no if on the platform, no separate build of your code. The same game.py that opens a window with python game.py is the one that becomes the web page.

API

Starting up

Loading

An anims entry looks like {"run": {"from": 0, "to": 8, "speed": 12, "loop": True}}.

Making things

Components

Events

Key names are "left", "right", "up", "down", "space", "enter", "escape", "tab", "shift", "ctrl", "alt", "backspace", plus "a"–"z" and "0"–"9".

Every event above takes a function, or sits above one as a decorator — including wait, loop, scene, obj.onGround and the state() handlers:

onKeyPress("space", jump)     # hand it a function
@onKeyPress("space")          # or put it above one
def jump(): ...

@onUpdate                     # no argument, so no parentheses
def every_frame(): ...

@onUpdate("enemy")            # once per object with that tag
def chase(enemy): ...

@loop(1.0)
def spawn_one(): ...

@scene("game")
def build_game(): ...

Values and helpers

Object methods

Every object has .destroy(), .exists(), .is_(tag), .pos, and .tags, plus whatever its components added.

Examples

All 13 lessons of the guide live in examples/ in this repository — they aren't part of the pip package, so clone the repo if you want to run them as they're written:

git clone https://github.com/sfranz2422/kaypy
cd kaypy/examples
python lesson10_levels.py

If you installed from pip instead, kaypy new mygame gives you every asset those lessons use — sprites from the package, sounds fetched on the spot — so you can follow the guide by typing its code into your own game.py.

Troubleshooting

The sprite is invisible but the game runs. Almost always a bad path. loadSprite raises on a missing file, so check the name you drew with matches the name you loaded.

Nothing collides. Both objects need area(). That's the single most common cause.

It falls through the floor. The floor needs area() and body(isStatic=True). With only area() it's a trigger, not a wall.

Every frame is half one pose and half the next. sliceX doesn't match the number of frames in the strip.

The animation is stuck on frame 0. .play() restarts from the first frame, so calling it every frame inside onKeyDown never lets it advance. Only call it when the animation should actually change.

AttributeError: 'GameObj' object has no attribute 'jump'. That object doesn't have body().

Press F1 while a game is running to draw every collision box.

Still stuck? Ask on the Discord, or open an issue. If you are teaching with this and something is in your way, that is worth saying out loud — most of what is in here came from a classroom.

Assets

examples/images/, examples/dungeon/, examples/dungeon.png and examples/sounds/ hold real sprites and sounds vendored from the KAPLAY project (MIT) and 0x72's DungeonTileset II (CC0). See examples/CREDITS.md for the full licence text and attribution. Only the files the lessons actually use are included, not the full packs.

examples/gen_assets.py fills in placeholder shapes for any sprite that isn't there yet, and never overwrites a real file.

How it works, and what's rough

DEVELOPING.md has the architecture, the test suite, the known limitations, and the war stories — including the three real physics bugs and the pygbag incompatibilities that the web export had to work around before it stopped using pygbag.

Credits

kaypy is an independent project. It is not affiliated with, endorsed by, or produced by the KAPLAY team. It follows KAPLAY's published API on purpose, so that KAPLAY's documentation and examples tell you what to write — but it is a separate implementation in Python, and bugs in it are mine, not theirs.

KAPLAY's API design, and its documentation, which this follows deliberately and closely. Built on pygame-ce; the web export runs on Pyodide. It was built on pygbag first, which is what made a browser build possible at all while this was finding its feet.

The bundled sprites and sounds come from KAPLAY (MIT) and 0x72's DungeonTileset II (CC0). Full terms travel with the files, in examples/CREDITS.md.