In this tutorial you will see how to do collision detection using rectangles. When testing if two things have collided you need to consider how accurate you want to be. In this set of examples we are doing collision detection with rectangles which means, as we have a ball, it may not always look smooth.

Some sample code to try –

from pygame import *

init()

width = 640
height = 480

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

animationTimer = time.Clock()
ball = Rect(100,200,40,40)
obstacle = Rect(270,190,100,100)
dx = 1 # how fast/direction in x 
dy = 4 # 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
	ball.move_ip(dx,dy)
	# border check
	if ball.y < 0 or ball.y > height - 40 :
		dy *= -1
	if ball.x < 0 or ball.x > width - 40:
		dx *= -1
	# check collision with obstacle
	if ball.colliderect(obstacle):
		if ball.centerx <= obstacle.x or ball.centerx>=obstacle.x +100:
			dx = dx * - 1 
		if ball.centery <= obstacle.y or ball.centery>=obstacle.y +100:
			dy = dy * - 1 
	# draw stuff
	screen.fill((100,100,200))
	# circle
	draw.ellipse(screen, (0,255,0), ball) 
	# draw obstacle
	draw.rect(screen, (200,200,200), obstacle)
	# limit to 30 frames per second 
	animationTimer.tick(100)
	# update the screen!
	display.update()

Helpful links

http://tech.pro/tutorial/1007/collision-detection-with-pygame

http://thepythongamebook.com/en:pygame:step018 – Using masks to perform collision detection (advanced)