字符串还支持 切片。索引可以提取单个字符,切片 则提取子字符串:

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

注意,输出结果包含切片开始,但不包含切片结束。因此,s[:i] + s[i:] 总是等于 s

>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

切片索引的默认值很有用;省略开始索引时,默认值为 0,省略结束索引时,默认为到字符串的结尾:

>>> word[:2]   # character from the beginning to position 2 (excluded)
'Py'
>>> word[4:]   # characters from position 4 (included) to the end
'on'
>>> word[-2:]  # characters from the second-last (included) to the end
'on'

还可以这样理解切片,索引指向的是字符 之间 ,第一个字符的左侧标为 0,最后一个字符的右侧标为 nn 是字符串长度。例如:

 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1

第一行数字是字符串中索引 0…6 的位置,第二行数字是对应的负数索引位置。ij 的切片由 ij 之间所有对应的字符组成。

对于使用非负索引的切片,如果两个索引都不越界,切片长度就是起止索引之差。例如, word[1:3] 的长度是 2。

索引越界会报错:

>>> word[42]  # the word only has 6 characters
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

但是,切片会自动处理越界索引:

>>> word[4:42]
'on'
>>> word[42:]
''