遍历整个列表

假设我们有一个名单，需要将其中每个名字打印出来。上一节，我们是分别获取每个元素的索引，然后打印名字。如果名单很长很长，就需要大量重复代码。此时可以用到for循环。

magicians = ['alice', 'david', 'carolina']
for magician in magicians:
    print(magician)

输出结果：

>>>
alice
david
carolina

我们定义了一个for循环for magician in magicians:，这行代码将从列表magicians中取出一个名字，存储在magician中；对于每一个magician，都执行print(magician)。这就是for循环。

深入地研究for循环

Python将首先读取第一行代码：

for magician in magicians:

这行代码让Python获取列表magicians中第一个值（'alice'），存储在magician中，然后Python读取下一行代码：

    print(magician)

Python打印magician的值，也就是'alice'。由于列表中还有其他元素，Python返回循环第一行。

for magician in magicians:

Python获取列表中下一个元素（'david'），再执行这行代码：

    print(magician)

直到列表中所有元素被打印为止。

编写for循环时，临时变量的取名可以是任何名称，但是建议可以取有意义的名称，例如：

for cat in cats:
for dog in dogs:
for item in list_of_items:

在for循环中执行更多操作

下面扩展前面的示例，对于每个magician执行更多的操作：夸他的表演很精彩。

magicians = ['alice', 'david', 'carolina']
for magician in magicians:
    print(magician.title() + ", that was a great trick!")
    print("I can't wait to see your next trick, " + magician.title() + ".\n")

上面的两个print都缩进了，也就是属于这个for循环，都将对magician执行一次。\n是换行符，会在迭代结束后，插入一个空行。

输出结果：

>>>
Alice, that was a great trick!
I can't wait to see your next trick, Alice.

David, that was a great trick!
I can't wait to see your next trick, David.

Carolina, that was a great trick!
I can't wait to see your next trick, Carolina.

在for循环结束后执行一些操作

for循环结束后，通常要总结性输出或接着执行其他任务。下面打印一条向全体致谢的消息。这条信息只打印一次，要放在for循环之外。

magicians = ['alice', 'david', 'carolina']
for magician in magicians:
    print(magician.title() + ", that was a great trick!")
    print("I can't wait to see your next trick, " + magician.title() + ".\n")
print("Thank you, everypne. That was a great show!")

输出结果：

>>>
Alice, that was a great trick!
I can't wait to see your next trick, Alice.

David, that was a great trick!
I can't wait to see your next trick, David.

Carolina, that was a great trick!
I can't wait to see your next trick, Carolina.

Thank you, everypne. That was a great show!

常见的错误

1. 忘记缩进。位于循环组成部分的代码行一定要缩进。
2. 忘记缩进额外的代码行。如果上文中的两个print只缩进了一个，结果也会不如意。
3. 不必要的缩进。如果最后一句总结性的打印也缩进了，就会循环输出。
4. 遗漏了冒号。for语句末尾是有冒号的。

练习题

4-1 比萨

想出至少三种你喜欢的比萨，将其名称存储在一个列表中，再使用for循环将每种比萨的名称都打印出来。

1. 修改这个for循环，使其打印包含比萨名称的句子，而不仅仅是比萨的名称。对于每种比萨，都显示一行输出，如“I like pepperoni pizza”。
2. 在程序末尾添加一行代码，它不在for循环中，指出你有多喜欢比萨。输出应包含针对每种比萨的消息，还有一个总结性句子，如“I really love pizza!”。

pizzas = ["french bread","baked ziti","grandma slice","new york slice","chicago pizza","puck's smoked salmon pizza"]
for pizza in pizzas:
    print(pizza)

for pizza in pizzas:
    print("I like " + pizza.title() + ".")
print("I really like pizza!")

输出结果：

french bread
baked ziti
grandma slice
new york slice
chicago pizza
puck's smoked salmon pizza
I like French Bread.
I like Baked Ziti.
I like Grandma Slice.
I like New York Slice.
I like Chicago Pizza.
I like Puck'S Smoked Salmon Pizza.
I really like pizza!

4-2 动物

想出至少三种有共同特征的动物，将这些动物的名称存储在一个列表中，再使用for循环将每种动物的名称都打印出来。

1. 修改这个程序，使其针对每种动物都打印一个句子，如“A dog would make a great pet”。
2. 在程序末尾添加一行代码，指出这些动物的共同之处，如打印诸如“Any of these animals would make a great pet!”这样的句子。

# 不写动物，写水果。最近有些缺乏维生素。
fruits = ["pineapples","apples","mongos"]
for fruit in fruits:
    print(fruit.title() + " are rich in vitamins.")
print("Any of these fruits is rich in vitamins")

输出结果：

Pineapples are rich in vitamins.
Apples are rich in vitamins.
Mongos are rich in vitamins.
Any of these fruits is rich in vitamins