Step 10 of 12

Move Bob with W A S D

Add this update method to the Bob class in player.py, between __init__ and draw

    def update(self, dt: float, keys, mouse_pos) -> None:
        direction = pygame.Vector2(
            keys[pygame.K_d] - keys[pygame.K_a],
            keys[pygame.K_s] - keys[pygame.K_w],
        )
        if direction.length_squared() > 0:
            direction.normalize_ip()
        self.pos += direction * BOB_SPEED * dt
        self.pos.x = max(BOB_RADIUS, min(WIDTH - BOB_RADIUS, self.pos.x))
        self.pos.y = max(BOB_RADIUS, min(HEIGHT - BOB_RADIUS, self.pos.y))

keys is a list of every key on the keyboard, and each one is True if it's being held down or False if not. Python treats True as 1 and False as 0, so we can do maths with them.

• Across: D minus A. If you hold D that is 1 - 0 = 1 (right). Hold A and it's 0 - 1 = -1 (left). Hold both and they cancel out to 0.
• Down: S minus W, the same trick. W is -1 because up is negative.
• So direction ends up as a little arrow telling us which way you want to go. Hold D and S together and it's (1, 1), down and to the right.
• There's a catch. (1, 1) is a longer arrow than (1, 0), so going diagonally would be faster than going straight. That's a sneaky bug lots of games have. normalize_ip() fixes it by squashing any arrow to a length of exactly 1, while keeping the direction. We only do that if the arrow isn't zero length, because you can't squash nothing.
• self.pos += direction * BOB_SPEED * dt is the actual move. Direction (which way) times speed (250 pixels per second) times dt (how many seconds this frame took). If a frame takes 1/60 of a second, Bob moves about 4 pixels. Using dt means he moves at the same speed on a fast computer or a slow one.
• The last two lines are a fence. min picks the smaller of two numbers and max picks the bigger, so together they trap x and y between the edges of the window. We leave a gap of BOB_RADIUS so the whole head stays in view.

The mouse_pos isn't used yet, we'll use it in the next step. In game.py we need to call update on every frame. Add this to step, right before the line that paints the background

        self.bob.update(dt, pygame.key.get_pressed(), pygame.mouse.get_pos())

pygame.key.get_pressed() hands over the list of keys and pygame.mouse.get_pos() gives the mouse position. Run the game and use W A S D. Bob should glide around and stop at the edges.