Efficient algorythm for checking occupied fields.

[Windspar]

Here a quick floodit example using NumPy and SciPy.

import pygame
import numpy
from scipy.ndimage import label as sci_label

# Simple Scene Interface
class Scene:
    def draw(self, surface, game): pass
    def event(self, event, game): pass
    def update(self, game): pass

class Game:
    def __init__(self, caption, width, height):
        # Basic pygame setup
        pygame.display.set_caption(caption)
        self.rect = pygame.Rect(0, 0, width, height)
        self.surface = pygame.display.set_mode(self.rect.size)
        self.clock = pygame.time.Clock()
        self.running = False
        self.fps = 30
        self.delta = 0

        # Scene Interface
        self.scene = Scene()
        Game.info = self

    def mainloop(self):
        self.running = True
        while self.running:
            for event in pygame.event.get():
                self.scene.event(event, self)

            self.keys = pygame.key.get_pressed()

            self.scene.update(self)
            self.scene.draw(self.surface, self)
            pygame.display.flip()
            self.delta = self.clock.tick(self.fps)

class FloodIt(Scene):
    def __init__(self):
        self.colors = tuple(map(pygame.Color,
            ['dodgerblue', 'gold', 'firebrick1', 'darkslateblue',
             'forestgreen', 'darkorange', 'mediumorchid'] ))

        self.surface = pygame.Surface((420, 420))
        self.new_game()

    def new_game(self):
        self.grid = numpy.random.randint(1, 8, size=(14, 14)).astype(numpy.uint8)
        self.update_grid()

    def draw(self, surface, game):
        surface.blit(self.surface, (190, 100))

    def event(self, event, game):
        if event.type == pygame.MOUSEBUTTONDOWN:
            if event.button == 1:
                x, y = event.pos
                if 190 < x < 610:
                    if 100 < y < 520:
                        self.selected_block((x - 190) // 30, (y - 100) // 30)
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE:
                self.new_game()
        elif event.type == pygame.QUIT:
            game.running = False

    def selected_block(self, x, y):
        if self.grid[0, 0] != self.grid[x, y]:
            value = self.grid[x, y]
            v = self.grid[0, 0]
            vl, vc = sci_label(self.grid == v)
            # Assume first group will always be right
            w = numpy.where(vl == 1)
            for x ,y in zip(*w):
                self.grid[x, y] = value

            self.update_grid()

    def update_grid(self):
        for x, row in enumerate(self.grid):
            for y, col in enumerate(row):
                self.surface.fill(self.colors[col - 1], (x * 30, y * 30, 29, 29))

def main():
    pygame.init()
    game = Game("FloodIt", 800, 600)
    game.scene = FloodIt()
    game.mainloop()
    pygame.quit()

main()