# Blocking Movements

Right now the walls don't do much; you can drive right through them! To prevent that, we'll need to check if the tank is colliding with the wall. Previously, we had used colliderect to detect collision. We can still use that to check if the tank collided with the wall pieces one-by-one, but since we have a big list of walls, it's easier to use collidelist instead.

If we collide with the wall when moving the tank, we will cancel the movement.

``````def update():
# Save the original position of the tank
original_x = tank.x
original_y = tank.y

# Move the tank if keys are pressed
if keyboard.left:
tank.x = tank.x - 2
tank.angle = 180
elif keyboard.right:
tank.x = tank.x + 2
tank.angle = 0
elif keyboard.up:
tank.y = tank.y - 2
tank.angle = 90
elif keyboard.down:
tank.y = tank.y + 2
tank.angle = 270

# Return tank to original position if colliding with wall
if tank.collidelist(walls) != -1:
tank.x = original_x
tank.y = original_y
``````

This is what the code does...

``````original_x = tank.x
original_y = tank.y
``````

This saves the original position of the tank into the original_x and original_y variable.

``````if tank.collidelist(walls) != -1:
tank.x = original_x
tank.y = original_y
``````

The tank.collidelist(walls) will return a number that tells us which piece of wall the tank is colliding with. If the tank isn't colliding with any pieces of wall, it will return -1. So if the value isn't -1, we know that a collision has occured, and we'll restore the original position before the move.

# Off Screen

Now our tank can't move through the walls anymore, but it can still drive off the screen! To prevent that, we need to check the position of the tank after movement, and return it back to the original position if it is off the screen.

``````if tank.x < 0 or tank.x > 800 or tank.y < 0 or tank.y > 600:
tank.x = original_x
tank.y = original_y
``````

Try it out.

You'll see that the tank can still move about halfway off screen. That's because the position of the tank is based on it's center. What can we do to prevent that? Think about it and attempt it on your own.