Collision detection without classes, in Pygame

I think you mean you don't want to create your own classes such as:

class Player(pygame.sprite.Sprite):
    def __init__(self, rect, image):
        pygame.sprite.Sprite.__init__(self)
        self.rect = rect
        self.image = image

Which is doable, but not a very good practice. With that in mind, here's how you do it.

Create a player sprite:

player = pygame.sprite.Sprite()
# size is 40x40, position is 0, 0
player.rect = pygame.Rect(0, 0, 40, 40)

# image is 40x40 grey block
player.image = pygame.Surface((40, 40))
player.image.fill((60, 60, 60))

Then add a block sprite that the player can collect:

blocks = pygame.sprite.Sprite()
# size is 20x20, position is 300, 300
block.rect = pygame.Rect(300, 300, 20, 20)

# image is 20x20 green block
block.image = pygame.Surface((20, 20))
block.image.fill((60, 200, 60))

With two sprites, pygame.sprite.collide_rect() can be used for collision detection:

if pygame.sprite.collide_rect(player, block):
    # move block to a new position
    block.rect.x = random.randint(20, 480)
    block.rect.y = random.randint(20, 480)
    print "You ate a block!"

The sprites can be drawn on the screen like this:

# topleft is the position of the top left corner of the sprite
screen.blit(block.image, block.rect.topleft)
screen.blit(player.image, player.rect.topleft)

Here's a fully working example.