Debugging short exercise

Objective: Practice debugging techniques

This program is supposed to break once the ball has been caught, but it doesn't work. Your job is to work out why, and then fix it.

1. Copy the program at the bottom of this page, and open it in Dr. Python
2. Add print statements to trace program execution, like this:

    print "Trace: At start"
   

   Add at least five statements:
   1. Start of the program
   2. Once the game window has been created
   3. Inside of the main game update loop
   4. Inside of the if statement that checks if the ball has been caught
   5. End of program
3. Print the distance before the 'if' statement (so that it runs on every iteration):

    print "Distance/ball to mitt: ", round(distance(ball_x, ball_y, mitt_x, mitt_y))
   

   
   '
4. Run the program and stop it as soon as you notice the buggy behavior.
5. Analyse the console output (the debug 'trace').
   Is the execution sequence what we would expect? (e.g. is it getting to the right place in the code at the right time?)
   Are the values what we would expect?
6. Once you've worked out what the problem is, fix it.
7. Name your file "MY_NAME_DEBUG_EX.txt" and then submit your code to school web lockers.
8. GOOD LUCK!

   
from gasp import *

def distance(x1, y1, x2, y2):
    return ((x2 + x1)**2 + (y2 + y1)**2)**0.5

begin_graphics(800, 600, title="Catch", background=color.YELLOW)
set_speed(120)

ball_x = 10
ball_y = 300
ball = Circle((ball_x, ball_y), 10, filled=True)

dx = 4
dy = random_between(-4, 4)

mitt_x = 740
mitt_y = 300
mitt = Circle((mitt_x, mitt_y), 30)

while True:
    # move the ball
    if ball_y >= 590 or ball_y <= 10:
        dy *= -1
        
    ball_x += dx
    if ball_x >= 810 or ball_x <= 10:
        dx *= -1
    
    # check on the mitt
    if key_pressed('k') and mitt_y <= 580:
        mitt_y += 5
    elif key_pressed('j') and mitt_y >= 20:
        mitt_y -= 5

    ball_y += dy    
    ball_x += dx
    move_to(ball, (ball_x, ball_y))

    if key_pressed('escape'):
        break

    move_to(mitt, (mitt_x, mitt_y))

    if distance(ball_x, ball_y, mitt_x, mitt_y) <= 30:  # ball is caught
        remove_from_screen(ball)
        break

    update_when('next_tick')

end_graphics()