drawing a line moves like a clock pointer in pygame -

i trying draw line moving in same way clock pointer , when run program line move 20 degrees program got frozen , , can read :"valueerror: math domain error"
here code have written :

import pygame import sys import time import math  pygame.locals import * pygame.init() windowsurface = pygame.display.set_mode((300, 600), 0, 32) pygame.display.set_caption("circle_line")  black = (0, 0, 0) white = (250, 250, 250)   r = 50 circle_line_start_x = 150 circle_line_start_y = 300 circle_line_end_x = circle_line_start_x circle_line_end_y = circle_line_start_y - r   move_speed = 3     while true:     event in pygame.event.get():          if event.type == quit:              pygame.quit()              sys.exit()        if circle_line_end_x <= circle_line_start_x , circle_line_end_y <= circle_line_start_y :         circle_line_end_x -= move_speed         circle_line_end_y = math.sqrt(r**2 - ((circle_line_end_x - circle_line_start_x)**2)) + circle_line_start_y      if circle_line_end_x <= circle_line_start_x , circle_line_end_y >= circle_line_start_y :         circle_line_end_x += move_speed         circle_line_end_y = math.sqrt(r**2 - ((circle_line_end_x - circle_line_start_x)**2)) + circle_line_start_y      if circle_line_end_x >= circle_line_start_x , circle_line_end_y >= circle_line_start_y :         circle_line_end_x += move_speed         circle_line_end_y = math.sqrt(r**2 - ((circle_line_end_x - circle_line_start_x)**2)) + circle_line_start_y      if circle_line_end_x >= circle_line_start_x , circle_line_end_y <= circle_line_start_y :         circle_line_end_x -= move_speed         circle_line_end_y = math.sqrt(r**2 - ((circle_line_end_x - circle_line_start_x)**2)) + circle_line_start_y         windowsurface.fill(white)      pygame.draw.line(windowsurface, black, (circle_line_start_x, circle_line_start_y), (circle_line_end_x, circle_line_end_y), 3)      pygame.display.update()     time.sleep(0.02) 

any find out mistake appreciated.

thank you.

this error because trying calculate square root of negative number, concretely result of r**2 - ((circle_line_end_x - circle_line_start_x)**2).

however, if want draw based on circular movement, suggest use variable called degrees , increment in each iteration. can use math.sin , math.cos calculate distance based on angle.