Question

In this part of the Tkinter tutorial, we create a Nibbles game clone.

Nibbles is an older classic video game. It was first created in late 70s. Later it was brought to PCs. In this game the player controls a snake. The objective is to eat as many apples as possible. Each time the snake eats an apple, its body grows. The snake must avoid the walls and its own body.

Development

The size of each of the joints of a snake is 10 px. The snake is controlled with the cursor keys. Initially, the snake has three joints. The game starts immediately. When the game is finished, we display "Game Over" message in the center of the window.

We use the Canvas widget to create the game. The objects in the game are images. We use canvas methods to create image items. We use canvas methods to find items on the canvas using tags and to do collision detection.

nibbles.py

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode Tkinter tutorial 

This is a simple Nibbles game
clone.

Author: Jan Bodnar
Website: zetcode.com 
Last edited: November 2015
"""

import sys
import random
from PIL import Image, ImageTk
from Tkinter import Tk, Frame, Canvas, ALL, NW


WIDTH = 300
HEIGHT = 300
DELAY = 100
DOT_SIZE = 10
ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 27

x = [0] * ALL_DOTS
y = [0] * ALL_DOTS


class Board(Canvas):

  def __init__(self, parent):
    Canvas.__init__(self, width=WIDTH, height=HEIGHT, 
      background="black", highlightthickness=0)
     
    self.parent = parent 
    self.initGame()
    self.pack()
             
  
  def initGame(self):

    self.left = False
    self.right = True
    self.up = False
    self.down = False
    self.inGame = True
    self.dots = 3
    
    self.apple_x = 100
    self.apple_y = 190

    for i in range(self.dots):
      x[i] = 50 - i * 10
      y[i] = 50
    
    try:
      self.idot = Image.open("dot.png")
      self.dot = ImageTk.PhotoImage(self.idot)  
      self.ihead = Image.open("head.png")
      self.head = ImageTk.PhotoImage(self.ihead)       
      self.iapple = Image.open("apple.png")
      self.apple = ImageTk.PhotoImage(self.iapple) 

    except IOError, e:
      print e
      sys.exit(1)

    self.focus_get()

    self.createObjects()
    self.locateApple()
    self.bind_all("<Key>", self.onKeyPressed)
    self.after(DELAY, self.onTimer)
    

  def createObjects(self):
  
    self.create_image(self.apple_x, self.apple_y, image=self.apple,
      anchor=NW, tag="apple")
    self.create_image(50, 50, image=self.head, anchor=NW,  tag="head")
    self.create_image(30, 50, image=self.dot, anchor=NW, tag="dot")
    self.create_image(40, 50, image=self.dot, anchor=NW, tag="dot")
   

  def checkApple(self):

    apple = self.find_withtag("apple")
    head = self.find_withtag("head")
    
    x1, y1, x2, y2 = self.bbox(head)
    overlap = self.find_overlapping(x1, y1, x2, y2)
      
    for ovr in overlap:
      
      if apple[0] == ovr:
        
        x, y = self.coords(apple)
        self.create_image(x, y, image=self.dot, anchor=NW, tag="dot")
        self.locateApple()
    
  
  def doMove(self):
    
    dots = self.find_withtag("dot")
    head = self.find_withtag("head")
        
    items = dots + head
    
    z = 0
    while z < len(items)-1:
      c1 = self.coords(items[z])
      c2 = self.coords(items[z+1])
      self.move(items[z], c2[0]-c1[0], c2[1]-c1[1])
      z += 1

    if self.left:
      self.move(head, -DOT_SIZE, 0)
      
    if self.right: 
      self.move(head, DOT_SIZE, 0)

    if self.up:
      self.move(head, 0, -DOT_SIZE)

    if self.down:
      self.move(head, 0, DOT_SIZE)
      

  def checkCollisions(self):

    dots = self.find_withtag("dot")
    head = self.find_withtag("head")
    
    x1, y1, x2, y2 = self.bbox(head)
    overlap = self.find_overlapping(x1, y1, x2, y2)
    
    for dot in dots:
      for over in overlap:
        if over == dot:
          self.inGame = False
      
    if x1 < 0:
      self.inGame = False
    
    if x1 > WIDTH - DOT_SIZE:
      self.inGame = False

    if y1 < 0:
      self.inGame = False
    
    if y1 > HEIGHT - DOT_SIZE:
      self.inGame = False
    

  def locateApple(self):
  
    apple = self.find_withtag("apple")
    self.delete(apple[0])
  
    r = random.randint(0, RAND_POS)
    self.apple_x = r * DOT_SIZE
    r = random.randint(0, RAND_POS)
    self.apple_y = r * DOT_SIZE
    
    self.create_image(self.apple_x, self.apple_y, anchor=NW,
      image=self.apple, tag="apple")
        
   
  def onKeyPressed(self, e): 
  
    key = e.keysym

    if key == "Left" and not self.right: 
      self.left = True
      self.up = False
      self.down = False
    

    if key == "Right" and not self.left:
      self.right = True
      self.up = False
      self.down = False
    

    if key == "Up" and not self.down:
      self.up = True
      self.right = False
      self.left = False
    

    if key == "Down" and not self.up: 
      self.down = True
      self.right = False
      self.left = False
      
      
  def onTimer(self):

    if self.inGame:
      self.checkCollisions()
      self.checkApple()
      self.doMove()
      self.after(DELAY, self.onTimer)
    else:
      self.gameOver()      
      
       
  def gameOver(self):

    self.delete(ALL)
    self.create_text(self.winfo_width()/2, self.winfo_height()/2, 
      text="Game Over", fill="white")      


class Nibbles(Frame):

  def __init__(self, parent):
    Frame.__init__(self, parent)
        
    parent.title('Nibbles')
    self.board = Board(parent)
    self.pack()


def main():

  root = Tk()
  nib = Nibbles(root)
  root.mainloop()  


if __name__ == '__main__':
  main()

First we will define some constants used in our game.

The WIDTH and HEIGHT constants determine the size of the Board. The DELAY constant determines the speed of the game. The DOT_SIZE is the size of the apple and the dot of the snake. The ALL_DOTS constant defines the maximum number of possible dots on the Board. The RAND_POS constant is used to calculate a random position of an apple.

x = [0] * ALL_DOTS
y = [0] * ALL_DOTS

These two arrays store the x and y coordinates of all possible joints of a snake.

The initGame() method initialises variables, loads images, and starts a timeout function.

try:
  self.idot = Image.open("dot.png")
  self.dot = ImageTk.PhotoImage(self.idot)  
  self.ihead = Image.open("head.png")
  self.head = ImageTk.PhotoImage(self.ihead)       
  self.iapple = Image.open("apple.png")
  self.apple = ImageTk.PhotoImage(self.iapple) 

except IOError, e:
  print e
  sys.exit(1)

In these lines, we load our images. There are three images in the Nibbles game: the head, the dot, and the apple.

self.createObjects()
self.locateApple()

The createObjects() method creates items on the canvas. The locateApple() puts an apple randomly on the canvas.

self.bind_all("<Key>", self.onKeyPressed)

We bind the keyboard events to the onKeyPressed() method. The game is controlled with keyboard cursor keys.

def createObjects(self):

  self.create_image(self.apple_x, self.apple_y, image=self.apple,
    anchor=NW, tag="apple")
  self.create_image(50, 50, image=self.head, anchor=NW,  tag="head")
  self.create_image(30, 50, image=self.dot, anchor=NW, tag="dot")
  self.create_image(40, 50, image=self.dot, anchor=NW, tag="dot")

In the createObjects() method, we create game objects on the canvas. These are canvas items. They are given initial x and y coordinates. The image parameter provides the image to be displayed. The anchor parameter is set to NW ; this way the coordinates of the canvas item are the top-left points of the items. This is important if we want to be able to display images next to the borders of the root window. Try to delete the anchor and see what happens. The tag parameter is used to identify items on the canvas. One tag may be used for multiple canvas items.

The checkApple() method checks if the snake has hit the apple object. If so, we add another snake joint and call the locateApple() .

apple = self.find_withtag("apple")
head = self.find_withtag("head")

The find_withtag() method finds an item on the canvas using its tag. We need two items: the head of the snake and the apple. Note that even if there is only one item with a given tag, the method returns a tuple. This is a case for the apple item. And later the apple item is accessed the following way: apple[0] .

x1, y1, x2, y2 = self.bbox(head)
overlap = self.find_overlapping(x1, y1, x2, y2)

The bbox() method returns the bounding box points of an item. The find_overlapping() method finds colliding items for the given coordinates.

for ovr in overlap:
  
  if apple[0] == ovr:
    x, y = self.coords(apple)
    self.create_image(x, y, image=self.dot, anchor=NW, tag="dot")
    self.locateApple()

If the apple collides with the head, we create a new dot item at the coordinates of the apple object. We call the locateApple() method, which deletes the old apple item from the canvas and creates and randomly positions a new one.

In the doMove() method we have the key algorithm of the game. To understand it, look at how the snake is moving. We control the head of the snake. We can change its direction with the cursor keys. The rest of the joints move one position up the chain. The second joint moves where the first was, the third joint where the second was etc.

z = 0
while z < len(items)-1:
  c1 = self.coords(items[z])
  c2 = self.coords(items[z+1])
  self.move(items[z], c2[0]-c1[0], c2[1]-c1[1])
  z += 1

This code moves the joints up the chain.

if self.left:
  self.move(head, -DOT_SIZE, 0)

Move the head to the left.

In the checkCollisions() method, we determine if the snake has hit itself or one of the walls.

x1, y1, x2, y2 = self.bbox(head)
overlap = self.find_overlapping(x1, y1, x2, y2)

for dot in dots:
  for over in overlap:
    if over == dot:
      self.inGame = False

We finish the game if the snake hits one of its joints with the head.

if y1 > HEIGHT - DOT_SIZE:
  self.inGame = False

We finish the game if the snake hits the bottom of the Board .

The locateApple() method locates a new apple randomly on the board and deletes the old one.

apple = self.find_withtag("apple")
self.delete(apple[0])

Here we find and delete the apple that was eaten by the snake.

r = random.randint(0, RAND_POS)

We get a random number from 0 to RAND_POS - 1.

self.apple_x = r * DOT_SIZE
...
self.apple_y = r * DOT_SIZE

These lines set the x and y coordinates of the apple object.

In the onKeyPressed() method of the Board class, we determine the keys that were pressed.

if key == "Left" and not self.right: 
  self.left = True
  self.up = False
  self.down = False

If we hit the left cursor key, we set left variable to true. This variable is used in the doMove() method to change coordinates of the snake object. Notice also that when the snake is heading to the right, we cannot turn immediately to the left.

def onTimer(self):

  if self.inGame:
    self.checkCollisions()
    self.checkApple()
    self.doMove()
    self.after(DELAY, self.onTimer)
  else:
    self.gameOver() 

Every DELAY ms, the onTimer() method is called. If we are in the game, we call three methods that build the logic of the game. Otherwise the game is finished. The timer is based on the after() method, which calls a method after DELAY ms only once. To repeatedly call the timer, we recursively call the onTimer() method.

def gameOver(self):

  self.delete(ALL)
  self.create_text(self.winfo_width()/2, self.winfo_height()/2, 
    text="Game Over", fill="white")   

If the game is over, we delete all items on the canvas. Then we draw "Game Over" in the center of the screen.

Figure: Nibbles

This was the Nibbles computer game created with the Tkinter library.