创建list
Python内置的一种数据类型是列表：list。list是一种有序的集合，可以随时添加和删除其中的元素。
比如，列出班里所有同学的名字，就可以用一个list表示：
>>> ['Michael', 'Bob', 'Tracy']
['Michael', 'Bob', 'Tracy']
list是数学意义上的有序集合，也就是说，list中的元素是按照顺序排列的。
构造list非常简单，按照上面的代码，直接用 [ ] 把list的所有元素都括起来，就是一个list对象。通常，我们会把list赋值给一个变量，这样，就可以通过变量来引用list：
>>> classmates = ['Michael', 'Bob', 'Tracy']
>>> classmates # 打印classmates变量的内容
['Michael', 'Bob', 'Tracy']
由于Python是动态语言，所以list中包含的元素并不要求都必须是同一种数据类型，我们完全可以在list中包含各种数据：
>>> L = ['Michael', 100, True]
一个元素也没有的list，就是空list：
>>> empty_list = []
任务
假设班里有3名同学：Adam，Lisa和Bart，他们的成绩分别是 95.5，85 和 59，请按照 名字, 分数, 名字, 分数... 的顺序按照分数从高到低用一个list表示，然后打印出来。
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alt="" />
按照索引访问list
由于list是一个有序集合，所以，我们可以用一个list按分数从高到低表示出班里的3个同学：
>>> L = ['Adam', 'Lisa', 'Bart']
那我们如何从list中获取指定第 N 名的同学呢？方法是通过索引来获取list中的指定元素。
需要特别注意的是，索引从 0 开始，也就是说，第一个元素的索引是0，第二个元素的索引是1，以此类推。
因此，要打印第一名同学的名字，用 L[0]:
>>> print L[0]
Adam
要打印第二名同学的名字，用 L[1]:
>>> print L[1]
Lisa
要打印第三名同学的名字，用 L[2]:
>>> print L[2]
Bart
要打印第四名同学的名字，用 L[3]:
>>> print L[3]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
报错了！IndexError意思就是索引超出了范围，因为上面的list只有3个元素，有效的索引是 0，1，2。
所以，使用索引时，千万注意不要越界。
任务
三名同学的成绩可以用一个list表示：
L = [95.5, 85, 59]
请按照索引分别打印出第一名、第二名、第三名、第四名的分数。
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倒序访问list
我们还是用一个list按分数从高到低表示出班里的3个同学：
>>> L = ['Adam', 'Lisa', 'Bart']
这时，老师说，请分数最低的同学站出来。
要写代码完成这个任务，我们可以先数一数这个 list，发现它包含3个元素，因此，最后一个元素的索引是2：
>>> print L[2]
Bart
有没有更简单的方法？
有！
Bart同学是最后一名，俗称倒数第一，所以，我们可以用 -1 这个索引来表示最后一个元素：
>>> print L[-1]
Bart
Bart同学表示躺枪。
类似的，倒数第二用 -2 表示，倒数第三用 -3 表示，倒数第四用 -4 表示：
>>> print L[-2]
Lisa
>>> print L[-3]
Adam
>>> print L[-4]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list index out of range
L[-4] 报错了，因为倒数第四不存在，一共只有3个元素。
使用倒序索引时，也要注意不要越界。
任务
三名同学的成绩可以用一个list表示：
L = [95.5, 85, 59]
请按照倒序索引分别打印出倒数第一、倒数第二、倒数第三、倒数第四的分数。
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alt="" />
添加新元素
现在，班里有3名同学：
>>> L = ['Adam', 'Lisa', 'Bart']
今天，班里转来一名新同学 Paul，如何把新同学添加到现有的 list 中呢？
第一个办法是用 list 的 append() 方法，把新同学追加到 list 的末尾：
>>> L = ['Adam', 'Lisa', 'Bart']
>>> L.append('Paul')
>>> print L
['Adam', 'Lisa', 'Bart', 'Paul']
append()总是把新的元素添加到 list 的尾部。
如果 Paul 同学表示自己总是考满分，要求添加到第一的位置，怎么办？
方法是用list的 insert()方法，它接受两个参数，第一个参数是索引号，第二个参数是待添加的新元素：
>>> L = ['Adam', 'Lisa', 'Bart']
>>> L.insert(0, 'Paul')
>>> print L
['Paul', 'Adam', 'Lisa', 'Bart']
L.insert(0, 'Paul') 的意思是，'Paul'将被添加到索引为 0 的位置上（也就是第一个），而原来索引为 0 的Adam同学，以及后面的所有同学，都自动向后移动一位。
任务
假设新来一名学生Paul，Paul 同学的成绩比Bart好，但是比Lisa差，他应该排到第三名的位置，请用代码实现。
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alt="" />
从list删除元素
Paul同学刚来几天又要转走了，那么我们怎么把Paul 从现有的list中删除呢？
如果Paul同学排在最后一个，我们可以用list的pop()方法删除：
>>> L = ['Adam', 'Lisa', 'Bart', 'Paul']
>>> L.pop()
'Paul'
>>> print L
['Adam', 'Lisa', 'Bart']
pop()方法总是删掉list的最后一个元素，并且它还返回这个元素，所以我们执行 L.pop() 后，会打印出 'Paul'。
如果Paul同学不是排在最后一个怎么办？比如Paul同学排在第三：
>>> L = ['Adam', 'Lisa', 'Paul', 'Bart']
要把Paul踢出list，我们就必须先定位Paul的位置。由于Paul的索引是2，因此，用 pop(2)把Paul删掉：
>>> L.pop(2)
'Paul'
>>> print L
['Adam', 'Lisa', 'Bart']
任务
注意右边编辑器代码中 list 如下：
L = ['Adam', 'Lisa', 'Paul', 'Bart']
Paul的索引是2，Bart的索引是3，如果我们要把Paul和Bart都删掉，请解释下面的代码为什么不能正确运行：
L.pop(2)
L.pop(3)
怎样调整代码可以把Paul和Bart都正确删除掉？
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jWlS71oAIliYuIOM9Kl3pQAYmLiGhd6VIPKiBxERGtK13qQQUkLiKidaVLPaiAxEVEtK50qQcVkLiIiNaVLvWgAhIXEdG60qUeVEDiIiJaV7rUgwpIXERE60qXelABiYuIaF3pUg8qIHEREa0rXepBBSQuIqJ1pUs9qIDERUS0rnSpBxX8Hynoncz9wIhjAAAAAElFTkSuQmCC" 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替换元素
假设现在班里仍然是3名同学：
>>> L = ['Adam', 'Lisa', 'Bart']
现在，Bart同学要转学走了，碰巧来了一个Paul同学，要更新班级成员名单，我们可以先把Bart删掉，再把Paul添加进来。
另一个办法是直接用Paul把Bart给替换掉：
>>> L[2] = 'Paul'
>>> print L
L = ['Adam', 'Lisa', 'Paul']
对list中的某一个索引赋值，就可以直接用新的元素替换掉原来的元素，list包含的元素个数保持不变。
由于Bart还可以用 -1 做索引，因此，下面的代码也可以完成同样的替换工作：
>>> L[-1] = 'Paul'
任务
班里的同学按照分数排名是这样的：
L = ['Adam', 'Lisa', 'Bart']
但是，在一次考试后，Bart同学意外取得第一，而Adam同学考了倒数第一。
请通过对list的索引赋值，生成新的排名。
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" alt="" />
创建tuple
tuple是另一种有序的列表，中文翻译为“ 元组 ”。tuple 和 list 非常类似，但是，tuple一旦创建完毕，就不能修改了。
同样是表示班里同学的名称，用tuple表示如下：
>>> t = ('Adam', 'Lisa', 'Bart')
创建tuple和创建list唯一不同之处是用( )替代了[ ]。
现在，这个 t 就不能改变了，tuple没有 append()方法，也没有insert()和pop()方法。所以，新同学没法直接往 tuple 中添加，老同学想退出 tuple 也不行。
获取 tuple 元素的方式和 list 是一模一样的，我们可以正常使用 t[0]，t[-1]等索引方式访问元素，但是不能赋值成别的元素，不信可以试试：
>>> t[0] = 'Paul'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
任务
创建一个tuple，顺序包含0 - 9这10个数。
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补充说明：如果采用range(0,10)函数产生的是序列，而非元组；
aaarticlea/png;base64,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" alt="" />
先列几个range（）函数的几个用法：
函数原型：range（start， end， scan):
参数含义：start:计数从start开始。默认是从0开始。例如range（5）等价于range（0， 5）;
end:技术到end结束，但不包括end.例如：range（0， 5） 是[0, 1, 2, 3, 4]没有5
scan：每次跳跃的间距，默认为1。例如：range（0， 5） 等价于 range(0, 5, 1)
创建单元素tuple
tuple和list一样，可以包含 0 个、1个和任意多个元素。
包含多个元素的 tuple，前面我们已经创建过了。
包含 0 个元素的 tuple，也就是空tuple，直接用 ()表示：
>>> t = ()
>>> print t
()
创建包含1个元素的 tuple 呢？来试试：
>>> t = (1)
>>> print t
1
好像哪里不对！t 不是 tuple ，而是整数1。为什么呢？
因为()既可以表示tuple，又可以作为括号表示运算时的优先级，结果 (1) 被Python解释器计算出结果 1，导致我们得到的不是tuple，而是整数 1。
正是因为用()定义单元素的tuple有歧义，所以 Python 规定，单元素 tuple 要多加一个逗号“,”，这样就避免了歧义：
>>> t = (1,)
>>> print t
(1,)
Python在打印单元素tuple时，也自动添加了一个“,”，为了更明确地告诉你这是一个tuple。
多元素 tuple 加不加这个额外的“,”效果是一样的：
>>> t = (1, 2, 3,)
>>> print t
(1, 2, 3)
任务
请指出右边编辑器中代码为什么没有创建出包含一个学生的 tuple：
t = ('Adam')
print t
请修改代码，确保 t 是一个tuple。
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“可变”的tuple
前面我们看到了tuple一旦创建就不能修改。现在，我们来看一个“可变”的tuple：
>>> t = ('a', 'b', ['A', 'B'])
注意到 t 有 3 个元素：'a'，'b'和一个list：['A', 'B']。list作为一个整体是tuple的第3个元素。list对象可以通过 t[2] 拿到：
>>> L = t[2]
然后，我们把list的两个元素改一改：
>>> L[0] = 'X'
>>> L[1] = 'Y'
再看看tuple的内容：
>>> print t
('a', 'b', ['X', 'Y'])
不是说tuple一旦定义后就不可变了吗？怎么现在又变了？
别急，我们先看看定义的时候tuple包含的3个元素：
当我们把list的元素'A'和'B'修改为'X'和'Y'后，tuple变为：
表面上看，tuple的元素确实变了，但其实变的不是 tuple 的元素，而是list的元素。
tuple一开始指向的list并没有改成别的list，所以，tuple所谓的“不变”是说，tuple的每个元素，指向永远不变。即指向'a'，就不能改成指向'b'，指向一个list，就不能改成指向其他对象，但指向的这个list本身是可变的！
理解了“指向不变”后，要创建一个内容也不变的tuple怎么做？那就必须保证tuple的每一个元素本身也不能变。
任务
定义了tuple：
t = ('a', 'b', ['A', 'B'])
由于 t 包含一个list元素，导致tuple的内容是可变的。能否修改上述代码，让tuple内容不可变？
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