Pygame线条转圆形动画开发问题：列表元素异常修改

我正在开发一个基于Pygame的Python程序，需求是先绘制线条，再将其动画转换为圆形，但目前连基础的线条绘制功能都存在问题。问题出在存储鼠标左键按下起点（列表第一个元素）和当前鼠标位置（第二个元素）的列表中，Python会错误地修改其中一个或两个元素的值。

以下是两种代码状态下的不同表现：

• 保留标记的两行更新代码时：程序每帧仅在鼠标当前位置绘制单个像素，无法画出从起点到当前位置的完整线条
• 删除这两行代码时：前一帧的线条不会被覆盖，每帧都会新增一条从起点到当前鼠标位置的线条

显然这两行代码是实现前一帧覆盖所必需的，我已在代码中用注释标记出影响结果的关键行，完整代码如下：

import pygame, PIL, random
print('\n')

#data
bubbles = []
color_options = [[87, 184, 222]]

pressed = False
released = False
bubline_start = []

background = [50, 25, 25]
size = [500, 500]

#pygame
display = pygame.display.set_mode(size)
pygame.init()

#functions
def new_bub_color():
    color_index = random.randint(0, len(color_options)-1)
    lvl = random.randrange(85, 115)

    bub_color = []
    for val in color_options[color_index]:
        bub_color.append(val*(lvl/100))
    return bub_color


def bubble_line():
    global display, pressed, bubline_start, released, bubbles, color_options
    
    if len(bubbles) > 0:
        if not bubbles[-1][0] == 0:
            #first frame of click
            bub_color = new_bub_color()

            bubbles.append([0, bub_color, [bubline_start, list(pygame.mouse.get_pos())]])
            pygame.draw.line(display, bub_color, bubline_start, pygame.mouse.get_pos())
        else:
            #draw after drags
            pygame.draw.line(display, bubbles[-1][1], bubbles[-1][2][0], list(pygame.mouse.get_pos()))            
            bubbles[-1][2][1] = list(pygame.mouse.get_pos())# <-- 关键行1
    else:
        #first bubble
        bub_color = new_bub_color()
        
        bubbles.append([0, bub_color, [bubline_start, list(pygame.mouse.get_pos())]])
        pygame.draw.line(display, bub_color, bubline_start, pygame.mouse.get_pos())

    if released:
        bubbles[-1][0] = 1
        bubbles[-1][2][1] = list(pygame.mouse.get_pos())# <-- 关键行2
        released = False


def cover_prev_frame():
    global bubbles, background, size
    min_pos = []
    max_pos = []

    for bubble in bubbles:
        min_pos = bubble[2][0]
        max_pos = bubble[2][0]

        for point in bubble[2]:
            #x min and max
            if point[0] < min_pos[0]:
                min_pos[0] = point[0]
            elif point[0] > max_pos[0]:
                max_pos[0] = point[0]

            #y min and max
            if point[1] < min_pos[1]:
                min_pos[1] = point[1]
            elif point[1] > max_pos[1]:
                max_pos[1] = point[1]
        max_pos = [max_pos[0]-min_pos[0]+1, max_pos[1]-min_pos[1]+1]

        if type(background) == str:
            #image background
            later = True

        elif type(background) == list:
            #solid color background
            pygame.draw.rect(display, background, pygame.Rect(min_pos, max_pos))


while True:
    pygame.event.pump()
    events = pygame.event.get()

    for event in events:
        if event.type == pygame.QUIT:
            pygame.quit()

        elif event.type == pygame.MOUSEBUTTONDOWN and not pressed:
            bubline_start = list(pygame.mouse.get_pos())
            pressed = True
            
        elif event.type == pygame.MOUSEBUTTONUP and pressed:
            pressed = False
            released = True

    cover_prev_frame()
    if pressed or released:
        bubble_line()

    try:
        pygame.display.update()
    except:
        break

内容的提问来源于stack exchange，提问作者killerderp7