Python中的字符串索引基本知识

Strings can be indexed (subscripted),
with the first character having index 0. There is no separate character type; a character is simply a string of size one:

>>> word = 'Python'
>>> word[0] # character in position 0
'P'
>>> word[5] # character in position 5
'n'

Indices may also be negative numbers, to start counting from the right:

>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]
'P'

Note that since -0 is the same as 0, negative indices start from -1.

In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters, slicing allows you to obtain substring:

>>> word[0:2] # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5] # characters from position 2 (included) to 5 (excluded)
'tho'Note how the start is always included, and the end always excluded.
This makes sure that 

s[:i] + s[i:]

 is
always equal to 

s

:

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

'Python'

Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second
index defaults to the size of the string being sliced.

>>> 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'Python strings cannot be changed — they are immutable.
Therefore, assigning to an indexed position in the string results in an error:

>>> word[0] = 'J'
...
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
...
TypeError: 'str' object does not support item assignment

The built-in function 

len()

 returns
the length of a string:

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34