import pygame
import random

def distance(pos1, pos2):
    # a and b are TEMPORARY VARIABLES.  They go away when the
    # function goes out of SCOPE.
    a = pos1[0] - pos2[0]
    b = pos1[1] - pos2[1]
    c = (a ** 2 + b ** 2) ** 0.5
    return c     # Send this value back to the caller

pygame.display.init()
pygame.font.init()
screen = pygame.display.set_mode((800, 600))
font = pygame.font.SysFont("Courier New", 16)
done = False

target_pos = (400, 300)

while not done:
    # INPUT
    evt = pygame.event.poll()
    if evt.type == pygame.QUIT:
        done = True
    elif evt.type == pygame.KEYDOWN:
        if evt.key == pygame.K_ESCAPE:
            done = True
    mpos = pygame.mouse.get_pos()

    # UPDATE
    d = int(distance(mpos, target_pos))
    mid_x = (mpos[0] + target_pos[0]) // 2
    mid_y = (mpos[1] + target_pos[1]) // 2

    # DRAW
    screen.fill((0,0,0))
    pygame.draw.line(screen, (255,255,255), mpos, target_pos)
    pygame.draw.circle(screen, (255,0,0), target_pos, 5)
    temps = font.render(str(d), False, (255,255,255), (32,32,32))
    screen.blit(temps, (mid_x - temps.get_width() // 2,
                        mid_y - temps.get_height() // 2))
    pygame.display.flip()

pygame.font.quit()
pygame.display.quit()