Question

In this part of the PyGTK programming tutorial, we will create a Snake game clone.

Snake game

Snake 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. This game is sometimes called Nibbles.

Development

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

snake.py

#!/usr/bin/python

# ZetCode PyGTK tutorial 
#
# This is a simple snake game
# clone
#
# author: jan bodnar
# website: zetcode.com 
# last edited: February 2009

import sys
import gtk
import cairo
import random
import glib


WIDTH = 300
HEIGHT = 270
DOT_SIZE = 10
ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 26

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


class Board(gtk.DrawingArea):

  def __init__(self):
    super(Board, self).__init__()

    self.modify_bg(gtk.STATE_NORMAL, gtk.gdk.Color(0, 0, 0))
    self.set_size_request(WIDTH, HEIGHT)

    self.connect("expose-event", self.expose)
 
    self.init_game()

  def on_timer(self):

    if self.inGame:
      self.check_apple()
      self.check_collision()
      self.move()
      self.queue_draw()
      return True
    else:
      return False
  
  def init_game(self):

    self.left = False
    self.right = True
    self.up = False
    self.down = False
    self.inGame = True
    self.dots = 3

    for i in range(self.dots):
      x[i] = 50 - i * 10
      y[i] = 50
    
    try:
      self.dot = cairo.ImageSurface.create_from_png("dot.png")
      self.head = cairo.ImageSurface.create_from_png("head.png")
      self.apple = cairo.ImageSurface.create_from_png("apple.png")
    except Exception, e:
      print e.message
      sys.exit(1)

    self.locate_apple()
    glib.timeout_add(100, self.on_timer)

    
    

  def expose(self, widget, event):
  
    cr = widget.window.cairo_create()

    if self.inGame:
      cr.set_source_rgb(0, 0, 0)
      cr.paint()

      cr.set_source_surface(self.apple, self.apple_x, self.apple_y)
      cr.paint()

      for z in range(self.dots):
        if (z == 0): 
          cr.set_source_surface(self.head, x[z], y[z])
          cr.paint()
        else:
          cr.set_source_surface(self.dot, x[z], y[z])         
          cr.paint()
    else:
      self.game_over(cr)
       
  

  def game_over(self, cr):

    w = self.allocation.width / 2
    h = self.allocation.height / 2

    (x, y, width, height, dx, dy) = cr.text_extents("Game Over")

    cr.set_source_rgb(65535, 65535, 65535)
    cr.move_to(w - width/2, h)
    cr.show_text("Game Over")
    self.inGame = False
  


  def check_apple(self):

    if x[0] == self.apple_x and y[0] == self.apple_y: 
      self.dots = self.dots + 1
      self.locate_apple()
    
  
  def move(self):

    z = self.dots

    while z > 0:
      x[z] = x[(z - 1)]
      y[z] = y[(z - 1)]
      z = z - 1

    if self.left:
      x[0] -= DOT_SIZE

    if self.right: 
      x[0] += DOT_SIZE

    if self.up:
      y[0] -= DOT_SIZE

    if self.down:
      y[0] += DOT_SIZE
    
  

  def check_collision(self):

    z = self.dots
     
    while z > 0:
      if z > 4 and x[0] == x[z] and y[0] == y[z]:
        self.inGame = False
      z = z - 1

    if y[0] > HEIGHT - DOT_SIZE: 
      self.inGame = False
    
    if y[0] < 0:
      self.inGame = False
    
    if x[0] > WIDTH - DOT_SIZE:
      self.inGame = False

    if x[0] < 0:
      self.inGame = False
    

  def locate_apple(self):
  
    r = random.randint(0, RAND_POS)
    self.apple_x = r * DOT_SIZE
    r = random.randint(0, RAND_POS)
    self.apple_y = r * DOT_SIZE
   

  def on_key_down(self, event): 
  
    key = event.keyval

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

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

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

    if key == gtk.keysyms.Down and not self.up: 
      self.down = True
      self.right = False
      self.left = False


class Snake(gtk.Window):

  def __init__(self):
    super(Snake, self).__init__()
    
    self.set_title('Snake')
    self.set_size_request(WIDTH, HEIGHT)
    self.set_resizable(False)
    self.set_position(gtk.WIN_POS_CENTER)

    self.board = Board()
    self.connect("key-press-event", self.on_key_down)
    self.add(self.board)
    
    self.connect("destroy", gtk.main_quit)
    self.show_all()


  def on_key_down(self, widget, event): 
   
    key = event.keyval
    self.board.on_key_down(event)


Snake()
gtk.main()

First we will define some globals used in our game.

The WIDTH and HEIGHT constants determine the size of the Board. 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. The DELAY constant determines the speed of the game.

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

These two lists store x, y coordinates of all possible joints of a snake.

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

self.left = False
self.right = True
self.up = False
self.down = False
self.inGame = True
self.dots = 3

When the game starts, the snake has three joints. And it is heading to the right.

In the move() method we have the key algorithm of the game. To understand it, look at how the snakeis moving. You control the head of the snake. You 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.

while z > 0:
  x[z] = x[(z - 1)]
  y[z] = y[(z - 1)]
  z = z - 1

This code moves the joints up the chain.

if self.left:
  x[0] -= DOT_SIZE

Move the head to the left.

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

while z > 0:
  if z > 4 and x[0] == x[z] and y[0] == y[z]:
    self.inGame = False
  z = z - 1

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

if y[0] > HEIGHT - DOT_SIZE: 
  self.inGame = False

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

The locate_apple() method locates an apple randomly on the form.

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 line set the x, y coordinates of the apple object.

  self.connect("key-press-event", self.on_key_down)
  ...

def on_key_down(self, widget, event): 
  
  key = event.keyval
  self.board.on_key_down(event)

We catch the key press event in the Snake class, and delegate the processing to the board object.

In the on_key_dow() method of the Board class, we determine which keys the player hit.

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

If we hit the left cursor key, we set self.left variable to True. This variable is used in the move() 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.

Figure: Snake

This was the Snake computer game programmed using PyGTK programming library.