Pygame - Limiting instances of sprites - pygame

Question

I'm using an example regarding sprite generation for a space scroller shootup that I'm developing. By slowly trying to understand how it works, I've managed to get multiple sprites to transverse across the screen. However, there are many sprites that are generated.
So what I'm having trouble with is limiting the initial number of sprites instead of the multitude that the code produces. I thought of using if sprites.__len__ < 10: sprites.add(drone) but when I tried that, it didn't work.
My thinking was that each time it looped, it would check the number of sprites in the group and if it was less then 10, add a sprite to the group until it hit 10. That way if it went off screen or is destroyed, then it would keep doing the check and keeping it constant.
This is the player class:
class Player(pygame.sprite.Sprite):
def __init__(self, *groups):
super(Player, self).__init__(*groups)
self.image = pygame.image.load('player.png')
self.rect = pygame.rect.Rect((screen_width, (random.randrange(0,screen_height))), self.image.get_size())
self.dx = -10
self.pos = random.randrange(0,screen_height)
def update(self):
self.rect.centerx += self.dx
if self.rect.right < 0:
self.kill()
and this is the section regarding the adding of the sprite.
sprites.update()
screen.fill((200, 200, 200))
sprites.draw(screen)
drone = Player()
self.y = random.randrange(0,screen_height)
sprites.add(drone)
pygame.display.flip()
It's probably obvious, but I'm still learning so guidance would be great.
Second question - More of a confirmation of thought. If I don't want the sprite to be half drawn on the bottom of the screen. Do I basically say that if self.rect.bottom > screen_height, then position the sprite # screen_height
Full source: http://pastebin.com/PLRVHtxz
EDIT - I think I've solved it, just need to make the sprites run smoother.
while 1:
clock.tick(40)
numberAlien = 5
for event in pygame.event.get():
if event.type == pygame.QUIT:
return
if event.type == pygame.KEYDOWN and event.key == pygame.K_ESCAPE:
return
sprites.update()
screen.fill((200, 200, 200))
sprites.draw(screen)
drone = Player()
if sprites.__len__() < numberAlien:
self.y = random.randrange(0,screen_height)
sprites.add(drone)
pygame.display.flip()

Answer 1

You could subclass SpriteGroup, add a new field of the total number of sprites, and check in the add method to see if it can be added.
You shouldn't test check any variables with __.
As for the movement, i believe, you do not see a smooth movement because of clock.tick(40).
It waits for 40ms until it resumes running. You could reduce tick to 10, and tune the dx you change for the sprites, or try a more universal approach.
A call to clock.tick() returns amount of ms since the last call. This will be your time delta. You can then have a global SPEED. The amount of pixels to be moved would be calculated from SPEED * delta.