Stopping my controlled object going off the screen in pygame - pygame

Question

i have started making a game in pygame using python 3.2.2
i have a title screen that has a new game button that when pressed a space background appears with a UFO you can control with the arrow keys
but my UFO can go off the screen in every direction,i want to make it stop on the border of the screen. and i want it to do this on every future screen that will appear later in the game.
i have no idea of the principles to do this...and where it should sit in the code. i read something about rectangles. do i need to define a rectangle then get the ufo to detect this rectangle and stop when it detects the rectangle
any help appreciated
im pretty good at working stuff out,so a push in the right direction would be appreciated.
thanks

Answer 1

What you need is collision detection. Here is some sample AABB collision detection code in python:
def collide(A_top, A_bot, A_left, A_right, B_top, B_bot, B_left, B_right):
if B_left < A_right and B_top < A_bot and B_right > A_left and B_bot > A_top:
return True #IS colliding
return False #IS NOT colliding
Now just collide with the borders of the screen, or anything else that happens to be on the screen.
OTHER SOLUTION:
Just check like this:
if ufo.x < 0 or ufo.y < 0 or ufo.x + ufo.width > screen.width or ufo.y + ufo.height > screen.height:
#Do collide code