A snake game in Python 3.3 with Pygame

nsnake.py

import sys,os,pygame,random,math

pygame.init()
pygame.display.set_caption("Nsnake v1.0")
pygame.font.init()
random.seed()

#Global constant definitions
SPEED = 0.36
SNAKE_SIZE = 9
APPLE_SIZE = SNAKE_SIZE
SEPARATION = 10
SCREEN_HEIGHT = 600
SCREEN_WIDTH = 800
FPS = 25
KEY = {"UP":1,"DOWN":2,"LEFT":3,"RIGHT":4}
#Screen initialization
screen = pygame.display.set_mode((SCREEN_WIDTH,SCREEN_HEIGHT),pygame.HWSURFACE)


#Resources
score_font = pygame.font.Font(None,38)
score_numb_font = pygame.font.Font(None,28)
game_over_font = pygame.font.Font(None,46)
play_again_font = score_numb_font
score_msg = score_font.render("Score:",1,pygame.Color("green"))
score_msg_size = score_font.size("Score")
#icon = pygame.image.load("nsnake32.png")
#pygame.display.set_icon(icon)

background_color = pygame.Color(74,74,74)
black = pygame.Color(0,0,0)

#Clock
gameClock = pygame.time.Clock()


def checkCollision(posA,As,posB,Bs):
    #As size of a | Bs size of B
    if(posA.x   < posB.x+Bs and posA.x+As > posB.x and posA.y < posB.y + Bs and posA.y+As > posB.y):
        return True
    return False

def checkLimits(entity):
    if(entity.x > SCREEN_WIDTH):
        entity.x = SNAKE_SIZE
    if(entity.x < 0):
        entity.x = SCREEN_WIDTH - SNAKE_SIZE
    if(entity.y > SCREEN_HEIGHT):
        entity.y = SNAKE_SIZE
    if(entity.y < 0):
        entity.y = SCREEN_HEIGHT - SNAKE_SIZE
        
class Apple:
    def __init__(self,x,y,state):
        self.x = x
        self.y = y
        self.state = state
        self.color = pygame.color.Color("red")
    def draw(self,screen):
        pygame.draw.rect(screen,self.color,(self.x,self.y,APPLE_SIZE,APPLE_SIZE),0)
        
class Segment:
    def __init__(self,x,y):
        self.x = x
        self.y = y
        self.direction = KEY["UP"]
        self.color = "white"
        
class Snake:
    def __init__(self,x,y):
        self.x = x
        self.y = y
        self.direction = KEY["UP"]
        self.stack = []
        
        self.stack.append(self)
        
        blackBox = Segment(self.x,self.y + SEPARATION)
        blackBox.direction = KEY["UP"]
        blackBox.color = "NULL"
        self.stack.append(blackBox)

        
        
    def move(self):
        last_element = len(self.stack)-1
        while(last_element != 0):
            self.stack[last_element].direction = self.stack[last_element-1].direction
            self.stack[last_element].x = self.stack[last_element-1].x 
            self.stack[last_element].y = self.stack[last_element-1].y 
            last_element-=1
        if(len(self.stack)<2):
            last_segment = self
        else:
            last_segment = self.stack.pop(last_element)
        last_segment.direction = self.stack[0].direction
        if(self.stack[0].direction ==KEY["UP"]):
            last_segment.y = self.stack[0].y - (SPEED * FPS)
        elif(self.stack[0].direction == KEY["DOWN"]):
            last_segment.y = self.stack[0].y + (SPEED * FPS) 
        elif(self.stack[0].direction ==KEY["LEFT"]):
            last_segment.x = self.stack[0].x - (SPEED * FPS)
        elif(self.stack[0].direction == KEY["RIGHT"]):
            last_segment.x = self.stack[0].x + (SPEED * FPS)
        self.stack.insert(0,last_segment)

    def getHead(self):
        return(self.stack[0])
    
    def grow(self):
        last_element = len(self.stack)-1
        self.stack[last_element].direction = self.stack[last_element].direction
        if(self.stack[last_element].direction == KEY["UP"]):
            newSegment = Segment(self.stack[last_element].x,self.stack[last_element].y-SNAKE_SIZE)
            blackBox = Segment(newSegment.x,newSegment.y-SEPARATION)
            
        elif(self.stack[last_element].direction == KEY["DOWN"]):
            newSegment = Segment(self.stack[last_element].x,self.stack[last_element].y+SNAKE_SIZE)
            blackBox = Segment(newSegment.x,newSegment.y+SEPARATION)
            
        elif(self.stack[last_element].direction == KEY["LEFT"]):
            newSegment = Segment(self.stack[last_element].x-SNAKE_SIZE,self.stack[last_element].y)
            blackBox = Segment(newSegment.x-SEPARATION,newSegment.y)
            
        elif(self.stack[last_element].direction == KEY["RIGHT"]):
            newSegment = Segment(self.stack[last_element].x+SNAKE_SIZE,self.stack[last_element].y)
            blackBox = Segment(newSegment.x+SEPARATION,newSegment.y)
            
        blackBox.color = "NULL"
        self.stack.append(newSegment)
        self.stack.append(blackBox)
        
    def iterateSegments(self,delta):
        pass
    
    def setDirection(self,direction):
        if(self.direction == KEY["RIGHT"] and direction == KEY["LEFT"] or self.direction == KEY["LEFT"] and direction == KEY["RIGHT"]):
            pass
        elif(self.direction == KEY["UP"] and direction == KEY["DOWN"] or self.direction == KEY["DOWN"] and direction == KEY["UP"]):
            pass
        else:
            self.direction = direction
            
    def get_rect(self):
        rect = (self.x,self.y)
        return rect
    
    def getX(self):
        return self.x
    
    def getY(self):
        return self.y
    
    def setX(self,x):
        self.x = x
        
    def setY(self,y):
        self.y = y
        
    def checkCrash(self):
        counter = 1
        while(counter < len(self.stack)-1):
            if(checkCollision(self.stack[0],SNAKE_SIZE,self.stack[counter],SNAKE_SIZE)and self.stack[counter].color != "NULL"):
                return True
            counter+=1
        return False
    
    def draw(self,screen):
        pygame.draw.rect(screen,pygame.color.Color("yellow"),(self.stack[0].x,self.stack[0].y,SNAKE_SIZE,SNAKE_SIZE),0)
        counter = 1
        while(counter < len(self.stack)):
            if(self.stack[counter].color == "NULL"):
                counter+=1
                continue
            pygame.draw.rect(screen,pygame.color.Color("white"),(self.stack[counter].x,self.stack[counter].y,SNAKE_SIZE,SNAKE_SIZE),0)
            counter+=1
        
                
def getKey():
        for event in pygame.event.get():
            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_UP:
                    return KEY["UP"]
                elif event.key == pygame.K_DOWN:
                    return KEY["DOWN"]
                elif event.key == pygame.K_LEFT:
                    return KEY["LEFT"]
                elif event.key == pygame.K_RIGHT:
                    return KEY["RIGHT"]
                elif event.key == pygame.K_ESCAPE:
                    return "exit"
                elif event.key == pygame.K_y:
                    return "yes"
                elif event.key == pygame.K_n:
                    return "no"
            if event.type == pygame.QUIT:
                sys.exit()

def respawnApple(apples,index,sx,sy):
    radius = math.sqrt((SCREEN_WIDTH/2*SCREEN_WIDTH/2  + SCREEN_HEIGHT/2*SCREEN_HEIGHT/2))/2
    angle = 999
    while(angle > radius):
        angle = random.uniform(0,800)*math.pi*2
        x = SCREEN_WIDTH/2 + radius * math.cos(angle)
        y = SCREEN_HEIGHT/2 + radius * math.sin(angle)
        if(x == sx and y == sy):
            continue
    newApple = Apple(x,y,1)
    apples[index] = newApple
        
def respawnApples(apples,quantity,sx,sy):
    counter = 0
    del apples[:]
    radius = math.sqrt((SCREEN_WIDTH/2*SCREEN_WIDTH/2  + SCREEN_HEIGHT/2*SCREEN_HEIGHT/2))/2
    angle = 999
    while(counter < quantity):
        while(angle > radius):
            angle = random.uniform(0,800)*math.pi*2
            x = SCREEN_WIDTH/2 + radius * math.cos(angle)
            y = SCREEN_HEIGHT/2 + radius * math.sin(angle)
            if( (x-APPLE_SIZE == sx or x+APPLE_SIZE == sx) and (y-APPLE_SIZE == sy or y+APPLE_SIZE == sy) or radius - angle <= 10):
                continue
        apples.append(Apple(x,y,1))
        angle = 999
        counter+=1
        
def endGame():
    message = game_over_font.render("Game Over",1,pygame.Color("white"))
    message_play_again = play_again_font.render("Play Again? Y/N",1,pygame.Color("green"))
    screen.blit(message,(320,240))
    screen.blit(message_play_again,(320+12,240+40))

    pygame.display.flip()
    pygame.display.update()
    
    myKey = getKey()
    while(myKey != "exit"):
        if(myKey == "yes"):
            main()
        elif(myKey == "no"):
            break
        myKey = getKey()
        gameClock.tick(FPS)
    sys.exit()

def drawScore(score):
    score_numb = score_numb_font.render(str(score),1,pygame.Color("red"))
    screen.blit(score_msg, (SCREEN_WIDTH-score_msg_size[0]-60,10) )
    screen.blit(score_numb,(SCREEN_WIDTH - 45,14))
    
def drawGameTime(gameTime):
    game_time = score_font.render("Time:",1,pygame.Color("green"))
    game_time_numb = score_numb_font.render(str(gameTime/1000),1,pygame.Color("red"))
    screen.blit(game_time,(30,10))
    screen.blit(game_time_numb,(105,14))
    
def exitScreen():
    pass

def main():
    score = 0

    #Snake initialization
    mySnake = Snake(SCREEN_WIDTH/2,SCREEN_HEIGHT/2)
    mySnake.setDirection(KEY["UP"])
    mySnake.move()
    start_segments=3
    while(start_segments>0):
        mySnake.grow()
        mySnake.move() 
        start_segments-=1

    #Apples
    max_apples = 1
    eaten_apple = False
    apples = [Apple(random.randint(60,SCREEN_WIDTH),random.randint(60,SCREEN_HEIGHT),1)]
    respawnApples(apples,max_apples,mySnake.x,mySnake.y)
    
    startTime = pygame.time.get_ticks()
    endgame = 0
    
    while(endgame!=1):
        gameClock.tick(FPS)

        #Input
        keyPress = getKey()
        if keyPress == "exit":
            endgame = 1
       
        #Collision check
        checkLimits(mySnake)
        if(mySnake.checkCrash()== True):
            endGame()
            
        for myApple in apples:
            if(myApple.state == 1):
                if(checkCollision(mySnake.getHead(),SNAKE_SIZE,myApple,APPLE_SIZE)==True):
                    mySnake.grow()
                    myApple.state = 0
                    score+=5
                    eaten_apple=True
            

        #Position Update
        if(keyPress):
            mySnake.setDirection(keyPress)    
        mySnake.move()
        
        
        
        #Respawning apples
        if(eaten_apple == True):
            eaten_apple = False
            respawnApple(apples,0,mySnake.getHead().x,mySnake.getHead().y)

        #Drawing
        screen.fill(background_color)
        for myApple in apples:
            if(myApple.state == 1):
                myApple.draw(screen)
                
        mySnake.draw(screen)
        drawScore(score)
        gameTime = pygame.time.get_ticks() - startTime
        drawGameTime(gameTime)
        
        pygame.display.flip()
        pygame.display.update()
        
       
        
        
        
main()

Its really a good game.
Great work .
You should add mew functionalities such as a start and end screen or a high score