Coding problem in a Pygame

[MTGReen]

In line 74 your "bullets, aliens"

def update_screen(ai_settings, screen, ship, stats, bullets, aliens, play_button):


should be reversed to "aliens, bullets,"

def update_screen(ai_settings, screen, ship, stats, aliens, bullets, play_button):

to match your sequence on lines

92 def update_bullets(ai_settings, screen, ship, aliens, bullets):

101 def check_bullet_alien_collision(ai_settings, screen, ship, aliens, bullets):

109 def update_aliens(ai_settings, stats, screen, ship, aliens, bullets):

131