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So we've now managed to create a snake body, move the snake,

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and we've even managed to put our snake related code into a separate class.

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Now the next step is to figure out how to control the snake.

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Now we're going to control the snake by using the up, down, left, and right arrow

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keys.

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And we're going to be using our key bindings that we learned in yesterday's

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lessons to be able to translate this into a change in direction of the snake. In

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our code right after we create our snake,

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we're going to call the screen.listen method to start listening for

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keystrokes.

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And the keystrokes that we're going to listen for are the up, down, left, and right

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arrow keys. Now those keys are described by a string.

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So it's a uppercase 'U' and then a 'p' so this would be up.

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And then it's again, uppercase 'Down', 'Left',

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and 'Right'.

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These are the keys that it will detect, obviously the up,

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down, left, and right arrow keys on the keyboard.

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The function that you're going to bind it to is going to be a function in the

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snake object and it's going to have the same name. So for example,

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it'd be snake.up,

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and that will be triggered when the up key is pressed.

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And then the same thing happens with snake.down when the down key gets

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pressed and then again with left and right.

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Your job is to have a think about how you can create these methods up,

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down, left, and right, in the snake class

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so that when the up key or the down key is detected,

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you can move the snake and change its heading accordingly.

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So remember that up is going to be towards North, down is going to be South, left

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is going to be towards the left of the screen and right is going to be towards

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the right of the screen.

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So have a think about the headings that we spoke about in previous lessons.

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And if you need to, it might be worth just setting up a brand new project,

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getting a turtle up and running and then setting it's heading from zero all the

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way to 360, just to see which direction it's actually facing.

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If you managed to complete the challenge,

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you should be able to control the snake

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and even though it's going to continue moving forward,

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you now determine which direction it moves in by using the arrow keys.

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Pause the video now and see if you can complete this challenge.

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All right. So I've already created all of these lines of code

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which is going to get the screen to listen for these key strokes.

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And then when it detects it,

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it's going to call these methods in the snake class.

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So inside our snake class, let's go ahead and create those methods.

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So we've got up,

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down,

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left, and right.

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Let's think about how we can turn the snake up. In order to turn the snake,

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the segment that we're most interested in is the first segment.

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That's basically the head of the snake.

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So we could tap into self.segments and then get hold of the zeroth segment,

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which is the first one, and then change its heading by calling setheading.

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And then we can tell it to, for example, if it's up,

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then it should actually be turning to the 90-degree direction,

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which is actually towards North.

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Now I'm going to go ahead and put a pass on all of these other methods,

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just so that we can actually test our code and it doesn't give us any errors

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because these functions are currently empty. Now, if you don't have the pass,

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it won't like it because Python doesn't like the idea of empty functions.

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But now we can at least just test this one thing.

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So when we run our code,

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we should be able to hit the up key and you can see the snake changed direction.

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Let me just do that again.

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And this is because we managed to get the first segment,

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the head of the snake, to change

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it's heading to 90 degrees and then the rest of the body will follow because

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it's continuously moving on every tick of the clock.

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So using this, we can now set the rest of the methods.

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But because we're going to need the head of the snake quite a few times in our

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code, it might make sense to actually create a separate attribute.

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So after we've created the snake, after we've got some segments inside this list,

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we can create an attribute called head,

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which is going to be the head of the snake.

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And this can be equal to self.segments at index zero. Now,

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remember if you have this line of code above the create_snake,

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it's probably going to error out because at this point the segments is empty

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and if you try to get the first item from it, it's not going to allow you to.

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So look at the positioning of the code here. Now, once we've got this set,

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we can go down here and change everywhere

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where we need to use the head of the snake. So for example,

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when we're moving the head forwards and when we're moving it up,

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left, down, and right. Now we can set

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each of these headings. Up is going to be a heading of 90, down is going to be 

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270,

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left is 180 and right is 0.

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These directions are just simply counter-clockwise.

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So the turtle starts out pointing towards East and that's a heading of zero,

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and then going anticlockwise, up is 90, left is 180, down as 270.

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This can be easily verified just by testing out some numbers, right?

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You could just put some numbers in there and then just try it out and see which

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direction it goes and then you'll be able to figure it out.

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But we've also mentioned this in previous lessons

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so you could also go back and take a look at when we looked at the heading of

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the turtle. So now if I go ahead and run this,

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then you should see that I can now freely control the snake by going left or

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right or down or up. And if you managed to do this,

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then you should consider yourself successful. Now,

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while I was testing this code,

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I realized that there's something about the snake game that we haven't really

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accounted for.

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And it's the fact that the snake can't move back on itself.

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It can't go forwards and then go the opposite direction because that requires

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the head to change directions.

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And this is not allowed in the official snake game.

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So how can we code this into our game? Well,

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we have to figure out when the head is pointing towards down direction,

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then we shouldn't allow it to go up. And similarly, when it's pointing up,

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we shouldn't let it go down, when it's pointing right

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we shouldn't let it go left. So at the top here,

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I'm going to create some constants.

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I'm going to create a UP direction and it's going to be 90, DOWN direction is going to

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be 270, LEFT is going to be equal to 180 and RIGHT is going to be equal to

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0. So now that I've got all of these as constants,

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I can go in here and set the heading to instead say UP when it needs to go up,

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DOWN, LEFT and RIGHT. In addition,

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we're going to do an if check.

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So we're going to say if the current self.head.heading

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is not equal to down, well, in that case,

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it's allowed to move up. So this way,

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it just means that if the current heading is pointed down,

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it can't move up. But for all other directions,

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if it's already moving up or left or right,

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then it can change the heading to up.

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Now be really careful here because it's not just self.head.heading,

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it's actually heading as a method because remember that the head of the snake is

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the first segment of our list of segments

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and each segment is a individual turtle.

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The turtle has a heading method

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which will give you a direction in terms of these 360-degree numbers,

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and then we can use that to check to see if it's equal to down.

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And if it is, then it's not allowed to go up.

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So using this logic,

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see if you can complete the rest of the three methods so that when you run your

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code, you can turn in all directions.

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But when you're facing one direction,

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you can't go backwards, like this.

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Okay?

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Alright. So it's going to be pretty straightforward.

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If it's already going up, then it's not allowed to go down.

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If it's already going right,

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then it's not allowed to go left. And finally,

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if it's already going left, then it's not allowed to go right.

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So this way, even though our snake can move freely through the space,

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it can't go back on itself. And there we have it.

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We've now managed to create our snake, get it to move and control it

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using our key presses.

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Even though it seems like our game is still not quite there,

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we've already come so far.

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So this comes up to our hour mark

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and it's now time to take a break, refresh your minds,

151
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look at some of the concepts that you might be confused about that we covered

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today,

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especially this concept of using the update to refresh the screen,

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as well as using the timer

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to delay the refresh so that we can control how often it happens.

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And then using our key bindings as well as separating all classes.

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We looked at a lot of things today. Now, tomorrow,

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once you're rested and fresh and ready,

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we're going to be finishing up the snake game.

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And we're going to be learning about inheritance as well as slicing

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and we're going to be building out the full functionality of this game.

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So I hope you're looking forward to it. I certainly am. I'll see you tomorrow.