Question

How do you make this code work? Just have pyglet installed and change "fireball.png" with the name of an image stored in the directory where you saved this code to a file.

import pyglet

class Fireball(pyglet.sprite.Sprite):
    def __init__(self, batch):
        pyglet.sprite.Sprite.__init__(self, pyglet.resource.image("fireball.png"))
        # replace "fireball.png" with your own image stored in dir of fireball.py
        self.x =  10 # Initial x coordinate of the fireball
        self.y =  10 # Initial y coordinate of the fireball

class Game(pyglet.window.Window):
    def __init__(self):
        pyglet.window.Window.__init__(self, width = 315, height = 220)
        self.batch_draw = pyglet.graphics.Batch()
        self.fps_display = pyglet.clock.ClockDisplay()
        self.fireball = []

    def on_draw(self):
        self.clear()
        self.fps_display.draw()
        self.batch_draw.draw()
        if len(self.fireball) != 0:             # Allow drawing of multiple
            for i in range(len(self.fireball)): # fireballs on screen
                self.fireball[i].draw()         # at the same time

    def on_key_press(self, symbol, modifiers):
        if symbol == pyglet.window.key.A:
            self.fireball.append(Fireball(batch = self.batch_draw))
            pyglet.clock.schedule_interval(func = self.update, interval = 1/60.)
            print "The 'A' key was pressed"

    def update(self, interval):
        for i in range(len(self.fireball)):
            self.fireball[i].x += 1 # why do fireballs get faster and faster?

if __name__ == "__main__":
    window = Game()
    pyglet.app.run()

This code creates a black background screen, where the fps are displayed and a fireball is shot along the x direction from the position (10, 10) whenever you press the A key.

You will notice that the more fireballs you shoot, the faster all fireballs will start to go.

Questions:

1. Why do the fireballs go faster and faster each time I press A ?

2. How should I stop the fireballs from accelerating each time I press A ?

Answer 1

The fireball goes faster and faster because every time you press the A you add another call of self.update to the scheduler. So self.update is called more and more times each time resulting in more updates of the position. To fix that move the line below to the __init__().

pyglet.clock.schedule_interval(func = self.update, interval = 1/60.)