'Bullet' object has no attribute '_Sprite__g' when adding Sprite to Group
<p>I wrote a simple game in Python and I got an  error.</p>

<pre><code>import pygame
import random
BLACK = (   0,   0,   0)
WHITE = ( 255, 255, 255)
RED   = ( 255,   0,   0)
BLUE  = (   0,   0, 255)
class Block(pygame.sprite.Sprite):
    def __init__(self, color):
        super().__init__()
        self.image = pygame.Surface([20, 15])
        self.image.fill(color)
        self.rect = self.image.get_rect()
class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.Surface([15, 20])
        self.image.fill(RED)
        self.rect = self.image.get_rect()
    def update(self):
        pos = pygame.mouse.get_pos()
        self.rect.x = pos[0]
class Bullet(pygame.sprite.Sprite):
    def __init__(self):
        self.image = pygame.Surface([4, 10])
        self.image.fill(BLACK)
        self.rect = self.image.get_rect()
    def update(self):
        self.rect.y -= 3
pygame.init()
screen_size = [700,500]
screen = pygame.display.set_mode(screen_size)
pygame.display.set_caption("Shooting things - Vers 2.0 of Blocks")
all_sprites_list = pygame.sprite.Group()
blocks_list = pygame.sprite.Group()
bullets_list = pygame.sprite.Group()
for i in range(50):
    block = Block(BLUE)
    block.rect.x = random.randrange( 660)
    block.rect.y = random.randrange( 480)

    blocks_list.add(block)
    all_sprites_list.add(block)
player = Player()
all_sprites_list.add(player)
score = 0
player.rect.y = 670
done = False
clock = pygame.time.Clock()
while not done:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            done = True
        if event.type == pygame.MOUSEBUTTONDOWN:
            bullet = Bullet()
            bullet.rect.x = player.rect.x
            bullet.rect.y = player.rect.y
            all_sprites_list.add(bullet)
            bullets_list.add(bullet)
    all_sprites_list.update()

    for bullet in bullets_list:
        block_hit_list = pygame.sprite.spritecollide(bullet, blocks_list, True)

        for block in block_hit_list:
            bullets_list.remove(bullet)
            all_sprites_list.remove(bullet)
            score += 1
            print(score)
        if bullet.rect.y &lt;-10:
            bullets_list.remove(bullet)
            all_sprites_list.remove(bullet)
    screen.fill(WHITE)
    all_sprites_list.draw(screen)
    pygame.display.update()
    clock.tick(60)
pygame.quit()
</code></pre>

<p>Here's the error:</p>

<pre><code>Traceback (most recent call last):
  File "C:\Documents and Settings\KHAI NHANH PC\Desktop\My Stuff\Python\Shooting things.py", line 57, in &lt;module&gt;
    all_sprites_list.add(bullet)
  File "C:\Python34\lib\site-packages\pygame\sprite.py", line 360, in add
    sprite.add_internal(self)
  File "C:\Python34\lib\site-packages\pygame\sprite.py", line 163, in add_internal
    self.__g[group] = 0
AttributeError: 'Bullet' object has no attribute '_Sprite__g'
</code></pre>

<p>I checked line 57 and I still don't know what went wrong.
Line 57: </p>

<blockquote>
<pre><code>all_sprites_list.add(bullet)
</code></pre>
</blockquote>