Pygame的屏幕不会变色

我有一个类似Agar.io的测试游戏，方形玩家触摸“位”并成长。问题是位的产生。该钻头每秒产生一次，并具有自己的表面。但是，程序不会将钻头（没有双关语）渗入初始表面。

这是代码：

import pygame
import random
pygame.init()
clock = pygame.time.Clock()
display = pygame.display
screen = display.set_mode([640,480])
rect_x = 295
rect_y = 215
display.set_caption("Agar")
d = False
bits = []
size = 50
steps = 0
class Bit:
    cscreen = display.set_mode([640,480])
    circ = pygame.draw.circle(cscreen, (65, 76, 150), (random.randrange(40, 600), random.randrange(40, 440)), 5)
while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            d = True
    if d == True:
        break

    # Fill background
    screen.fill((5, 58, 0))

    # Create player object
    player = pygame.draw.rect(screen, (250, 197, 255), [rect_x, rect_y, size, size])

    # Moving
    if pygame.key.get_pressed()[pygame.K_UP]:
        rect_y -= 2
    if pygame.key.get_pressed()[pygame.K_DOWN]:
        rect_y += 2
    if pygame.key.get_pressed()[pygame.K_RIGHT]:
        rect_x += 2
    if pygame.key.get_pressed()[pygame.K_LEFT]:
        rect_x -= 2

    # Bit generation
    if steps == 60:
        new_bit = Bit()
        bits.append(new_bit.circ)
        screen.blit(new_bit.cscreen, (0,0))
        steps = 0
    steps += 1

    # Collision detection
    collision = player.collidelist(bits)
    if collision != -1:
        bits[collision].cscreen.fill((0,0,0,0))
        # Make player larger
        size += 10

    # FPS limit
    clock.tick(60)

    # Refresh
    display.flip()

PS没有错误消息。

提前致谢。