Pythonでさめがめを作った

pythonの勉強としてゲーム作ってみようということでpygameを使って簡単に作ってみた。音とか点数計算はできません。また詰んで進めなくなっても終了判定されない雑な作りです。
プログラミングも才能無いけどとりあえず手を動かしていく。

コード

samegame.py

#!/usr/bin/env python

import random
import math
from functools import reduce
import pygame
from pygame.locals import *

length = 600
height = 400
ballSize = 20
vBallNum = int(height/(2*ballSize))
hBallNum = int(length/(2*ballSize))

WHITE = (255,255,255)
RED   = (255,0,0)
YELLOW = (255,255,0)
GREEN = (0, 128,0)
BLUE  = (0, 0, 255)
CYAN  = (0,255,255)
BLACK = (0,0,0)

balls = [RED, YELLOW, GREEN, BLUE, CYAN]

screen = pygame.display.set_mode((length,height))

board = [[random.choice(balls) for y in range(vBallNum)] for x in range(hBallNum)]

def drawBalls(bd):
    for x, cs in enumerate(bd):
        for y, c in enumerate(cs):
            pygame.draw.circle(screen,c,(ballSize*(2*x+1), ballSize*((vBallNum - y - 1)*2+1)),ballSize-1)
    return 0

def pickBall(ps,bd):
    print(*ps)
    for p in ps:
        if isValidPos(p,bd): del bd[p[0]][p[1]]
    align(bd)
    return None

def align(bd):        
    for x in reversed([x for x, c in enumerate(bd) if len(c) == 0]):
        del bd[x]
    return None

def selectBall(p):
    x = math.floor(p[0]/(2*ballSize))
    y = vBallNum - 1 - math.floor(p[1]/(2*ballSize))
    return (x,y)

def isValidPos(p, bd):
    if p == None:            return False
    if p[0] < 0 or p[1] < 0: return False
    if len(bd) == 0:         return False
    try:
        bd[p[0]][p[1]]
    except IndexError:  return False
    else:               return True

def markBall(bd,pos):
    if not isValidPos(pos,bd): return []
    ball = bd[pos[0]][pos[1]]
    def marking(p,stck):
        if not isValidPos(p,bd): return stck
        if p in stck:            return stck
        if ball == bd[p[0]][p[1]]:
            stck.append(p)
            return reduce(lambda stk, xy: marking(xy,stk),[(p[0],p[1]+1),(p[0],p[1]-1),(p[0]+1,p[1]),(p[0]-1,p[1])],stck)
        return stck
    return marking(pos,[])

pygame.init()

while True:
    screen.fill(BLACK)
    drawBalls(board)
    pygame.display.update()
    for event in pygame.event.get():
        if event.type == MOUSEBUTTONDOWN and event.button == 1:
            print(event.pos)
            ms = markBall(board,selectBall(event.pos))
            if len(ms) > 1: 
                ms.sort(reverse=True)
                print(ms)
                pickBall(ms,board)
        if event.type == QUIT:
            pygame.quit()
            sys.exit()