def draw_pixel(position):
    global SIZE
    global COLOR
    SCREEN.fill(COLOR, (position, SIZE))
while True:  # main loop
    pos = pygame.mouse.get_pos()
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        if event.type == MOUSEBUTTONDOWN:
            FLAG = True
        if event.type == MOUSEBUTTONUP:
            FLAG = False
    if FLAG:
        draw_pixel(pos)
    change_brush_size_button.draw_button()
    pygame.display.update()

 def draw_line(x1, y1, x2, y2):
    delta_x = abs(x2 - x1)
    delta_y = abs(y2 - y1)
    if x1 < x2:
        sign_x = 1
    else:
        sign_x = -1
    if y1 < y2:
        sign_y = 1
    else:
        sign_y = -1
    
    error = delta_x - delta_y
    
    draw_pixel([x2, y2])
    while (x1 != x2 or y1 != y2): 
        draw_pixel([x1, y1])
        error_2 = error * 2
        
        if error_2 > -delta_y: 
            error -= delta_y
            x1 += sign_x
        
        if error_2 < delta_x:
            error += delta_x
            y1 += sign_y

 while True:  # main loop
    pos = pygame.mouse.get_pos()
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
        if event.type == MOUSEBUTTONDOWN:
            flag = True
        if event.type == MOUSEBUTTONUP:
            flag = False
            
    if flag:
       new_pos = pygame.mouse.get_pos()
       draw_line(pos[0], pos[1], new_pos[0], new_pos[1])
        
   pygame.display.flip()

 FLAG = True