Pygame text problem

import pygame
import random
from math import *
from pygame.locals import *

win_flags = 0
screen_rect = Rect(0, 0, 800, 600)
n_stars = 200
n_asteroids = 10

# rgb colours
black = (0, 0, 0)
white = (255, 255, 255)
yellow = (255, 255, 0)
transparent = (1, 2, 3)

...

def main():
    # initialize pygame
    pygame.init()

    #initialize game font
    game_font = pygame.font.Font(None, 30)

    # create a new window
    screen = pygame.display.set_mode(screen_rect.size, win_flags)
    pygame.display.set_caption('Asteroids by Luca') # title of the game
    # create star map on the background
    background = pygame.Surface(screen_rect.size)
    background.fill(black)
    for i in range(0, n_stars):
        x = random.randint(0, screen_rect.width-1)
        y = random.randint(0, screen_rect.height-1)
        pygame.draw.circle(background, white, (x, y), 0)


    # copy the background onto the screen
    screen.blit(background, (0, 0))
    pygame.display.update()

    # initialize starting sprites
    player = Player()

    # initialize sprite groups
    render = pygame.sprite.Group()
    render.add(player)

    clock = pygame.time.Clock()

    # create asteroids
    asteroids = pygame.sprite.Group()
    for i in range(0, n_asteroids):
        # new asteroids are stored in the asteroids group
        asteroid = Asteroid()
        render.add(asteroid)
        asteroids.add(asteroid)

    lives = 3
    # lives text
    lives_text = game_font.render('%s' % lives, 2, white)
    lives_rect = lives_text.get_rect(center= (30, 30))
    screen.blit(lives_text, lives_rect)

    while True:
        ...

        # detect collisions
        for asteroid in pygame.sprite.spritecollide(player, asteroids, False):
            asteroid.kill()
            lives -= 1
            lives_text = game_font.render('%s' % lives, 2, white)
            lives_rect = lives_text.get_rect(center= (30, 30))
            screen.blit(lives_text, lives_rect)
            if lives == 0:
                player.kill()
                for asteroid in asteroids:
                        asteroid.kill()
                text = game_font.render('GAME OVER', False, white)
                text_rect = text.get_rect(center=screen_rect.center)
                screen.blit(text, text_rect)


        clock.tick(30)
        # update the sprites
        render.update()
        # draw the scene
        render.clear(screen, background)
        render.draw(screen)
        pygame.display.update()

# run main()
if __name__ == "__main__": main()