Extending pygame sprite class

Question

So I'm writing a game in python with pygame, and I have separate classes (of the type pygame.sprite.Sprite) for my different sprites - however they all share a lot of common physics code. How can I extend the basic sprite class, so that common physics stuff is written once, and I just add class specific stuff to each sprite class needed?

e.g. Change from this:

class ShipSprite(pygame.sprite.Sprite):

    def __init__(self, start_position=(500,500)):
        # Call the sprite initialiser
        pygame.sprite.Sprite.__init__(self)

        init_stuff()

    def common_physics_stuff()
        pass

    def ship_specific_stuff()
        pass

class AsteroidSprite(pygame.sprite.Sprite):

    def __init__(self, start_position=(500,500)):
        # Call the sprite initialiser
        pygame.sprite.Sprite.__init__(self)

        init_stuff()

    def common_physics_stuff()
        pass

    def asteroid_specific_stuff()
        pass

Into this

class my_custom_class()
    def common_physics_stuff()
        pass

class ShipSprite(my_custom_class):

    def __init__(self, start_position=(500,500)):
        # Call the sprite initialiser
        pygame.sprite.Sprite.__init__(self)

        init_stuff()

    def ship_specific_stuff()
        pass

class AsteroidSprite(my_custom_class):

    def __init__(self, start_position=(500,500)):
        # Call the sprite initialiser
        pygame.sprite.Sprite.__init__(self)

        init_stuff()

    def asteroid_specific_stuff()
        pass

Answer 1

Just inherit your intermediate class from Sprite, and inherit from it thereafter:

class my_custom_class(pygame.sprite.Sprite):
    def common_physics_stuff()
        pass

class ShipSprite(my_custom_class):
    ...

If you would like to add your "custom_class" things to a somewhat abstract class that does not have to behave like a Sprite, and could be used in other contexts, you could make use of Multiple Inheritance as well -

class my_custom_class(object):
    def common_physics_stuff()
        pass

class ShipSprite(pygame.sprite.Sprite, my_custom_class):
    ...

But that would probably be overkill - in any of these two cases, on any method you override on your game classes, just remember to call the proper ancestor method uisng the super Python builtin.

(On my small game projects I usually do a "GameObject" base class for all my objects which inherit from Pygame's Sprite)