The player object has a counter called 'pellets'.  By default, the game counts the number of pellets the player has collected and displays the count.

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To check if the player has eaten all the pellets, add the following line before the main while loop:

total_pellets = len(pellet_sprites)

Then, use the following lines inside the while loop:

if player.pellets >= total_pellets:
   game_over = True

The above two lines will end the game when all the pellets have been eaten.  You can replace 'game_over = True' with whatever you want to happen instead.