Simple snake game

snake.py

import random
import sys
import time

import pygame

BACKGROUND_COLOR = (0, 0, 0)
SNAKE_COLOR = (0, 255, 0)
FOOD_COLOR = (255, 0, 0)
BORDER_COLOR = (0, 0, 255)

WIDTH = 800
HEIGHT = 600

GRID_WIDTH = 80
GRID_HEIGHT = 50
GRID_CELL_WIDTH = int(WIDTH / GRID_WIDTH)
GRID_CELL_HEIGHT = int(HEIGHT / GRID_HEIGHT)

EXISTING_FOOD = 100
GROW_LENGTH = 5

# Update speeds
EASY = .1
MEDIUM = .05
HARD = .025
LUNATIC = .001
LEVEL = MEDIUM

def main():
    
    pygame.init()
    display = pygame.display.set_mode((WIDTH, HEIGHT))
    snakeCells = [(random.randint(0, WIDTH), random.randint(0, HEIGHT))] # [0] is the tail, end is the head
    food = []
    direction = (0, 0)
    nextDirection = (0, 0)
    lastUpdate = 0
    snakeGrowLength = 0
    
    while True:
        
        currentTime = time.time()
        
        # Event handling 
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                terminate()
            elif event.type == pygame.KEYDOWN:
                if event.key in (pygame.K_UP, pygame.K_w) and direction != (0, 1):
                    nextDirection = (0, -1)
                elif event.key in (pygame.K_LEFT, pygame.K_a) and direction != (1, 0):
                    nextDirection = (-1, 0)
                elif event.key in (pygame.K_DOWN, pygame.K_s) and direction != (0, -1):
                    nextDirection = (0, 1)
                elif event.key in (pygame.K_RIGHT, pygame.K_d) and direction != (-1, 0):
                    nextDirection = (1, 0)

        if len(food) < EXISTING_FOOD:
            food.append(newFoodLocation(snakeCells))
        
        if currentTime - lastUpdate >= LEVEL:
            lastUpdate = currentTime
            snakeGrowLength = moveSnake(snakeCells, food, direction, snakeGrowLength)
            direction = nextDirection

        # Rendering
        display.fill(BACKGROUND_COLOR)
        for x, y in food:
            cellRect = (x * GRID_CELL_WIDTH, y * GRID_CELL_HEIGHT, GRID_CELL_WIDTH, GRID_CELL_HEIGHT)
            pygame.draw.rect(display, FOOD_COLOR, cellRect)            
        for x, y in snakeCells:
            cellRect = (x * GRID_CELL_WIDTH, y * GRID_CELL_HEIGHT, GRID_CELL_WIDTH, GRID_CELL_HEIGHT)
            pygame.draw.rect(display, SNAKE_COLOR, cellRect)
        pygame.display.update()

def terminate():
    pygame.quit()
    sys.exit()

def newFoodLocation(snakeCells):
    x = random.randint(0, GRID_WIDTH - 1)
    y = random.randint(0, GRID_HEIGHT - 1)
    while (x, y) in snakeCells:        
        x = random.randint(0, GRID_WIDTH - 1)
        y = random.randint(0, GRID_HEIGHT - 1)
    return (x, y)

def moveSnake(snakeCells, food, direction, lengthToGrow):
    currentHead = snakeCells[len(snakeCells) - 1]
    if lengthToGrow == 0:
        del snakeCells[0]
    else:
        lengthToGrow -= 1
    x = (currentHead[0] + direction[0]) % GRID_WIDTH
    y = (currentHead[1] + direction[1]) % GRID_HEIGHT
    newHead = (x, y)
    if newHead in snakeCells:
        terminate()
        return
    snakeCells.append(newHead)
    if newHead in food:
        food.remove(newHead)
        lengthToGrow += GROW_LENGTH
    return lengthToGrow

if __name__ == '__main__':
    main()