ai_game为什么为指向alien的实例？

为什么ai_game是指向alien invasion的实例呢？
ship.py

import pygame

class Ship:
"""A class to manage the ship."""

def __init__(self,ai_game):
    """Initialize the ship and set its starting position."""
    self.screen = ai_game.screen
    self.screen_rect = ai_game.screen.get_rect()

    # Load the ship image and get its rect.
    self.image = pygame.image.load('images/ship.bmp')
    self.rect = self.image.get_rect()

    # Start each new ship at the bottom center of the screen.
    self.rect.midbottom = self.screen_rect.midbottom

def blitme(self):
    """Draw the ship at its current location."""
    self.screen.blit(self.image, self.rect)

alien.py

import pygame

from settings import Settings
from ship import Ship

class AlienInvasion:
"""Overall class to manage game assets and behavior."""

def __init__(self):
    """Initialize the game, and create game resources."""
    pygame.init()
    self.settings = Settings()

    self.screen = pygame.display.set_mode(
        (self.settings.screen_width, self.settings.screen_height))
    pygame.display.set_caption("Alien Invasion")

    self.ship = Ship(self)


def run_game(self):
    """Start the main loop for the game."""
    while True:
        # Watch for keyboard and mouse events.
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()

        # Redraw the screen during each pass through the loop.
        self.screen.fill(self.settings.bg_color)
        self.ship.blitme()

        # Make the most recently drawn screen visible.
        pygame.display.flip()

if name == 'main':
# Make a game instance, and run the game.
ai = AlienInvasion()
ai.run_game()