切片
-A slicing selects a range of items in a sequence object (e.g., a string, tuple or list). Slicings may be used as expressions or as targets in assignment or del statements.
- 简单切片
s[begin : end]
begin和end均可以为负值。-1是序列s最后一个元素的索引，-2是倒数第二个元素的索引，以此类推。
2.扩展切片
s[begin : end : stride]
当stride参数为正值时，从左至右访问源序列s的元素;当stride参数为负值时，从右至左访问源序列s的元素.
spam = ['cat', 'dog', 'bat', 'mouse', 'fish']
print(spam[-3:-1])
print(spam[:4])
print(spam[:-1])
print(spam[-3:])
print(spam[2:])
print(spam[: : -1])
print(spam[1:3:-1])
print(spam[3:1:-1])
输出 ['bat', 'mouse']
['cat', 'dog', 'bat', 'mouse']
['cat', 'dog', 'bat', 'mouse']
['bat', 'mouse', 'fish']
['bat', 'mouse', 'fish']
['fish', 'mouse', 'bat', 'dog', 'cat']
[]#逆向访问[1:3]之间无元素，返回空
['mouse', 'bat']#逆向访问[3:1]3为闭，1为开，访问不到'dog'
