Python -- 4. if()操作

1. 条件测试
每条if 语句的核心都是一个值为True 或False 的表达式，这种表达式被称为条件测试 。

• ==, !=, <, <=, >, >=
• and, or
• 检查特定值是否包含在列表中, in / not in

banned_users = ['andrew', 'carolina', 'david']user = 'marie'if user not in banned_users:    print(user.title() + ", you can post a response if you wish.")

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2. if 语句
(1).简单的if 语句

age = 19if age >= 18:    print("You are old enough to vote!")

(2).if-else 语句

age = 17if age >= 18:    print("You are old enough to vote!")    print("Have you registered to vote yet?")else:    print("Sorry, you are too young to vote.")    print("Please register to vote as soon as you turn 18!")

(3).if-elif-else 结构

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

(4).使用多个elif 代码块

age = 12if age < 4:    price = 0elif age < 18:    price = 5elif age < 65:    price = 10else:    price = 5print("Your admission cost is $" + str(price) + ".")

(5).省略else 代码块
Python并不要求if-elif 结构后面必须有else 代码块。

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3. 使用if 语句处理列表
确定列表不是空的
在if 语句中将列表名用在条件表达式中时，Python将在列表至少包含一个元素时返回True ，并在列表为空时返回False 。

requested_toppings = []if requested_toppings:    for requested_topping in requested_toppings:        print("Adding " + requested_topping + ".")    print("\nFinished making your pizza!")else:    print("Are you sure you want a plain pizza?")