Edge detection

When dealing with edge detection you must manually add it to your programs. Pygame does offer collision detection, which will be covered later, but simple screen boundary checks can be done using 2 if statements. By adding the x and y speeds (shown as dx and dy), you can easily manipulate them by reversing the sign (multiply by -1).

from pygame import *

init()

width = 640
height = 480

screen = display.set_mode((width, height)) # tuple
display.set_caption('Graphics')

animationTimer = time.Clock()
x=10
y=10
dx = 2 # how fast/direction in x 
dy = 3 # how fast/direction in y
endProgram = False

# game loops
# check input
# Update positions / game logic
# draw frame

while not endProgram:
	# Check input
	for e in event.get():
		if e.type == QUIT:
			endProgram = True
	# update position
	x += dx
	y += dy
	# border check
	if y<0 or y>height - 40 :
		dy *= -1
	if x<0 or x>width - 40:
		dx *= -1
	# draw stuff
	screen.fill((100,100,200))
	# circle
	draw.ellipse(screen, (0,255,0), (x, y, 40, 40)) 
	
	# limit to 30 frames per second 
	animationTimer.tick(100)
	# update the screen!
	display.update()