用户输入和while循环
函数input() ：让程序暂停运行，等待用户输入一些文本。接收1个参数：即要向用户显示的提示或说明。
1 >>> message = input("Tell me something, and I will repeat it back to you: ")
2 >>> print(message)
3 Tell me something, and I will repeat it back to you: Hello everyone!
4 Hello everyone!
函数int() ：将括号内的参数字符串转换为数值。
1 >>> age = input("How old are you? ")
2 >>> print(age)
3
4 >>> age = int(age)
5 >>> print(age >= 18)
6 How old are you? 21
7 21
8 True
求模运算符（%）：将两个数相除并返回余数。
while循环：
使用break语句退出循环。
1 >>> prompt = "\nPlease enter the name of a city you have visited: "
2 >>> prompt += "\n(Enter 'quit' when you are finished.)"
3 >>> while True:
4 >>> city = input(prompt)
5 >>> if city == 'quit':
6 >>> break
7 >>> else:
8 >>> print("I'd love to go to " + city.title() + "!")
9 Please enter the name of a city you have visited:
10 (Enter 'quit' when you are finished.)New York
11 I'd love to go to New York!
12
13 Please enter the name of a city you have visited:
14 (Enter 'quit' when you are finished.)quit
使用continue语句：不再执行余下的代码，返回到循环的开头并根据条件测试结果决定是否继续执行循环。
1 >>> current_number = 0
2 >>> while current_number < 10:
3 >>> current_number += 1
4 >>> if current_number % 2 == 0:
5 >>> continue
6 >>> print(current_number)
7 1
8 3
9 5
10 7
11 9
使用while循环来处理列表和字典；
1 >>> pets = ['peg', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
2 >>> print(pets)
3 >>> while 'cat' in pets:
4 >>> pets.remove('cat')
5 >>> print(pets)
6 ['peg', 'cat', 'dog', 'goldfish', 'cat', 'rabbit', 'cat']
7 ['peg', 'dog', 'goldfish', 'rabbit']
