do all class functions need to be called to be used - python-3.x

Question

I'm new to this so forgive me.
In the follow code on_draw(), on_update(), on_key_press(), and on_key_release() are never called but I know the code works so they do something, but why does it work?
I was under the impression that you had to do something like Mygame.on_update() to use a class function.
class Player(arcade.Sprite):
""" Player class """
def __init__(self, image, scale):
""" Set up the player """
# Call the parent init
super().__init__(image, scale)
# Create a variable to hold our speed. 'angle' is created by the parent
self.speed = 0
def update(self):
# Convert angle in degrees to radians.
angle_rad = math.radians(self.angle)
# Rotate the ship
self.angle += self.change_angle
# Use math to find our change based on our speed and angle
self.center_x += -self.speed * math.sin(angle_rad)
self.center_y += self.speed * math.cos(angle_rad)
class MyGame(arcade.Window):
"""
Main application class.
"""
def __init__(self, width, height, title):
"""
Initializer
"""
# Call the parent class initializer
super().__init__(width, height, title)
# Set the working directory (where we expect to find files) to the same
# directory this .py file is in. You can leave this out of your own
# code, but it is needed to easily run the examples using "python -m"
# as mentioned at the top of this program.
file_path = os.path.dirname(os.path.abspath(__file__))
os.chdir(file_path)
# Variables that will hold sprite lists
self.player_list = None
# Set up the player info
self.player_sprite = None
# Set the background color
arcade.set_background_color(arcade.color.BLACK)
def setup(self):
""" Set up the game and initialize the variables. """
# Sprite lists
self.player_list = arcade.SpriteList()
# Set up the player
self.player_sprite = Player(":resources:images/space_shooter/playerShip1_orange.png",
SPRITE_SCALING)
self.player_sprite.center_x = SCREEN_WIDTH / 2
self.player_sprite.center_y = SCREEN_HEIGHT / 2
self.player_list.append(self.player_sprite)
def on_draw(self):
"""
Render the screen.
"""
# This command has to happen before we start drawing
self.clear()
# Draw all the sprites.
self.player_list.draw()
def on_update(self, delta_time):
""" Movement and game logic """
# Call update on all sprites (The sprites don't do much in this
# example though.)
self.player_list.update()
def on_key_press(self, key, modifiers):
"""Called whenever a key is pressed. """
# Forward/back
if key == arcade.key.UP:
self.player_sprite.speed = MOVEMENT_SPEED
elif key == arcade.key.DOWN:
self.player_sprite.speed = -MOVEMENT_SPEED
# Rotate left/right
elif key == arcade.key.LEFT:
self.player_sprite.change_angle = ANGLE_SPEED
elif key == arcade.key.RIGHT:
self.player_sprite.change_angle = -ANGLE_SPEED
def on_key_release(self, key, modifiers):
"""Called when the user releases a key. """
if key == arcade.key.UP or key == arcade.key.DOWN:
self.player_sprite.speed = 0
elif key == arcade.key.LEFT or key == arcade.key.RIGHT:
self.player_sprite.change_angle = 0
def main():
""" Main function """
window = MyGame(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)
window.setup()
arcade.run()
if __name__ == "__main__":
main()