First we will define some constants used in our game.
class Cons:
BOARD_WIDTH = 300
BOARD_HEIGHT = 300
DELAY = 100
DOT_SIZE = 10
MAX_RAND_POS = 27
The BOARD_WIDTH and BOARD_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 MAX_RAND_POS constant is used to calculate a random position of an apple.
The initGame method initializes variables, loads images, and starts a timeout function.
self.createObjects() self.locateApple() The createObjects method creates items on the canvas. The locateApple puts an apple randomly on the canvas.
self.bind_all("", self.onKeyPressed) We bind the keyboard events to the onKeyPressed method. The game is controlled with keyboard cursor keys.
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 as e:
print(e)
sys.exit(1)
In these lines, we load our images. There are three images in the Snake game: the head, the dot, and the apple.
def createObjects(self): '''creates objects on Canvas'''
self.create_text(30, 10, text="Score: {0}".format(self.score),
tag="score", fill="white")
self.create_image(self.appleX, self.appleY, 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 checkAppleCollision method checks if the snake has hit the apple object. If so, we increase score, 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 moveSnake 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.
self.move(head, self.moveX, self.moveY) We move the head with the move method. The self.moveX and self.moveY variables are set when the cursor keys are pressed.
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 > Cons.BOARD_HEIGHT - Cons.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, Cons.MAX_RAND_POS) We get a random number from 0 to MAX_RAND_POS - 1.
self.appleX = r * Cons.DOT_SIZE ... self.appleY = r * Cons.DOT_SIZE These lines set the x and y coordinates of the apple object.
In the onKeyPressed method we react to the pressed keys during the game.
LEFT_CURSOR_KEY = "Left" if key == LEFT_CURSOR_KEY and self.moveX <= 0:
self.moveX = -Cons.DOT_SIZE
self.moveY = 0
If we hit the left cursor key, we set the self.moveX and self.moveY variables accordingly. These variables are used in the moveSnake 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): '''creates a game cycle each timer event '''
self.drawScore()
self.checkCollisions()
if self.inGame:
self.checkAppleCollision()
self.moveSnake()
self.after(Cons.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 drawScore(self): '''draws score'''
score = self.find_withtag("score")
self.itemconfigure(score, text="Score: {0}".format(self.score))
The drawScore method draws score on the board.
def gameOver(self): '''deletes all objects and draws game over message'''
self.delete(ALL)
self.create_text(self.winfo_width() /2, self.winfo_height()/2,
text="Game Over with score {0}".format(self.score), fill="white")
If the game is over, we delete all items on the canvas. Then we draw game over with the final score in the center of the screen.
Snake Figure: Snake This was the Snake computer game created with Tkinter.