python【列表1--基础---取值，增加，修改】

1、用下标取得列表的值

>>> spam = ['cat', 'bat', 'rat', 'elephant'] 
>>> spam[0] 
'cat' 
>>> spam[1] 
'bat' 
>>> spam[2] 
'rat' 
>>> spam[3] 
'elephant' 
>>> ['cat', 'bat', 'rat', 'elephant'][3] 
'elephant' 
>>> 'Hello ' + spam[0] 
'Hello cat' 
>>> 'The ' + spam[1] + ' ate the ' + spam[0] + '.' 
'The bat ate the cat.' 

列表也可以包含其他列表值。这些列表的列表中的值，可以通过多重下标来访问

>>> spam = [['cat', 'bat'], [10, 20, 30, 40, 50]] 
>>> spam[0] 
['cat', 'bat'] 
>>> spam[0][1] 
'bat' 
>>> spam[1][4] 
50 

2、负数下标

>>> spam = ['cat', 'bat', 'rat', 'elephant'] 
>>> spam[-1] 
'elephant' 
>>> spam[-3] 
'bat' 
>>> 'The ' + spam[-1] + ' is afraid of the ' + spam[-3] + '.' 
'The elephant is afraid of the bat.'

3、利用切片获得子列表
在一个切片中，第一个整数是切片开始处的下标。第二个整数是切片结束处的下标。切片向上增长，直至第二个下标的值，但不包括它。切片求值为一个新的列表值。

>>> spam = ['cat', 'bat', 'rat', 'elephant'] 
>>> spam[0:4] 
['cat', 'bat', 'rat', 'elephant'] 
>>> spam[1:3] 
['bat', 'rat'] 
>>> spam[0:-1] 
['cat', 'bat', 'rat']

作为快捷方法，你可以省略切片中冒号两边的一个下标或两个下标。省略第一个下标相当于使用 0，或列表的开始。省略第二个下标相当于使用列表的长度，意味着分片直至列表的末尾。

>>> spam = ['cat', 'bat', 'rat', 'elephant'] 
>>> spam[:2] 
['cat', 'bat'] 
>>> spam[1:] 
['bat', 'rat', 'elephant'] 
>>> spam[:] 
['cat', 'bat', 'rat', 'elephant']

4、获取列表的长度len()

>>> spam = ['cat', 'dog', 'moose'] 
>>> len(spam) 
3 

5、用索引改变列表的值
修改列表其中的某个值

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> spam[1] = 'aardvark'
>>> spam
['cat', 'aardvark', 'rat', 'elephant']
>>> spam[2] = spam[1]
>>> spam
['cat', 'aardvark', 'aardvark', 'elephant']
>>> spam[-1] = 12345
>>> spam
['cat', 'aardvark', 'aardvark', 12345]

列表的连接和复制

>>> [1, 2, 3] + ['A', 'B', 'C']
[1, 2, 3, 'A', 'B', 'C']
>>> ['X', 'Y', 'Z'] * 3
['X', 'Y', 'Z', 'X', 'Y', 'Z', 'X', 'Y', 'Z']
>>> spam = [1, 2, 3]
>>> spam = spam + ['A', 'B', 'C']
>>> spam
[1, 2, 3, 'A', 'B', 'C']

6、列表的删除

>>> spam = ['cat', 'bat', 'rat', 'elephant']
>>> del spam[2]
>>> spam
['cat', 'bat', 'elephant']
>>> del spam[2]
>>> spam
['cat', 'bat']