1. 用 in 和 not in 检查特定值是否包含在列表中：

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    requested_toppings = ['mushrooms', 'onions', 'pineapple']
    print('mushrooms' in requested_toppings)  # True
    print('pepperoni' in requested_toppings)  # False
     
    banned_users = ['andrew', 'carolina', 'david']
    user = 'marie'
     
    if user not in banned_users:
        print(user.title() + ", you can post a response if you wish.")

  1. 下面这段程序 str() 是必须的：

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    age = 12
    if age < 4:
        price = 0
    elif age < 18:
        price = 5
    else:
        price = 10
     
    print("Your admission cost is $" + str(price) + ".")

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