Pygame太空射手射击子弹

Question

我正在用Python 2.7制作自己的太空射击/侵略者游戏，我想使用空格键使其发射子弹。谁能帮我写出射击子弹的代码？ （下面的代码只是移动了船，而没有让它离开屏幕）

代码：

# Starting point
x, y = 250, 650
MOVE_RIGHT = 1
MOVE_LEFT = 2
direction = 0

circle_radius = 7
circle_pos = (int(x), int(y))

while True:
    for event in pygame.event.get():   
        if event.type == QUIT:
            exit()

        # movement     
        if event.type == KEYDOWN:
            if event.key == K_LEFT:
                direction = MOVE_LEFT
            elif event.key == K_RIGHT:
                direction = MOVE_RIGHT

        elif event.type == KEYUP:
            if event.key == K_LEFT:
                direction = 0
            elif event.key == K_RIGHT:
                direction = 0

    if(direction == MOVE_LEFT):
        x-=0.3
    elif(direction == MOVE_RIGHT):
        x+=0.3

    # out of boundaries
    if x > 517 - (circle_radius * 2):
        x = -x
        x = 517 - (circle_radius * 2)
    elif x < 0:
        x = -x
        x = 0

    screen.blit(background, (0, 0))
    screen.blit(player, (x, y))
    pygame.display.update()