python文档：字符串
3.1.2. 字符串 除了数字，Python 也可以操作字符串。字符串有多种形式，可以使用单引号（’…’），双引号（"…"）都可以获得同样的结果 。反斜杠 \ 可以用来转义:
>>> 'spam eggs' # single quotes
'spam eggs'
>>> 'doesn\'t' # use \' to escape the single quote...
"doesn't"
>>> "doesn't" # ...or use double quotes instead
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'在交互式解释器中，输出的字符串外面会加上引号，特殊字符会使用反斜杠来转义。 虽然有时这看起来会与输入不一样（外面所加的引号可能会改变），但两个字符串是相同的。 如果字符串中有单引号而没有双引号，该字符串外将加双引号来表示，否则就加单引号。 print() 函数会生成可读性更强的输出，即略去两边的引号，并且打印出经过转义的特殊字符:
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'
>>> print('"Isn\'t," they said.')
"Isn't," they said.
>>> s = 'First line.\nSecond line.' # \n means newline
>>> s # without print(), \n is included in the output
'First line.\nSecond line.'
>>> print(s) # with print(), \n produces a new line
First line.
Second line.如果你不希望前置了 \ 的字符转义成特殊字符，可以使用 原始字符串 方式，在引号前添加 r 即可:
>>> print('C:\some\name') # here \n means newline!
C:\some
ame
>>> print(r'C:\some\name') # note the r before the quote
C:\some\name字符串字面值可以跨行连续输入。一种方式是用三重引号："""…""" 或 ‘’’…’’’。字符串中的回车换行会自动包含到字符串中，如果不想包含，在行尾添加一个 \ 即可。如下例:
print("""\
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
""")将产生如下输出（注意最开始的换行没有包括进来）：
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to字符串可以用 + 进行连接（粘到一起），也可以用 * 进行重复:
>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'相邻的两个或多个 字符串字面值 （引号引起来的字符）将会自动连接到一起.
>>> 'Py' 'thon'
'Python'把很长的字符串拆开分别输入的时候尤其有用:
>>> text = ('Put several strings within parentheses '
... 'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'只能对两个字面值这样操作，变量或表达式不行:
>>> prefix = 'Py'
>>> prefix 'thon' # can't concatenate a variable and a string literal
File "", line 1
prefix 'thon'
^
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
File "", line 1
('un' * 3) 'ium'
^
SyntaxError: invalid syntax如果你想连接变量，或者连接变量和字面值，可以用 + 号:
>>> prefix + 'thon'
'Python'字符串是可以被 索引 （下标访问）的，第一个字符索引是 0。单个字符并没有特殊的类型，只是一个长度为一的字符串:
>>> word = 'Python'
>>> word[0] # character in position 0
'P'
>>> word[5] # character in position 5
'n'索引也可以用负数，这种会从右边开始数:
>>> word[-1] # last character
'n'
>>> word[-2] # second-last character
'o'
>>> word[-6]
'P'注意 -0 和 0 是一样的，所以负数索引从 -1 开始。
除了索引，字符串还支持 切片。索引可以得到单个字符，而 切片 可以获取子字符串
>>> word[0:2] # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5] # characters from position 2 (included) to 5 (excluded)
'tho'注意切片的开始总是被包括在结果中，而结束不被包括。这使得 s[:i] + s[i:] 总是等于s
>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'切片的索引有默认值；省略开始索引时默认为0，省略结束索引时默认为到字符串的结束:
>>> 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'您也可以这么理解切片：将索引视作指向字符 之间 ，第一个字符的左侧标为0，最后一个字符的右侧标为 n ，其中 n 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第一行数标注了字符串 0…6 的索引的位置，第二行标注了对应的负的索引。那么从 i 到 j 的切片就包括了标有 i 和 j 的位置之间的所有字符。
对于使用非负索引的切片，如果索引不越界，那么得到的切片长度就是起止索引之差。例如， word[1:3] 的长度为2。
试图使用过大的索引会产生一个错误:
>>> word[42] # the word only has 6 characters
Traceback (most recent call last):
File "", line 1, in <module>
IndexError: string index out of range但是，切片中的越界索引会被自动处理:
>>> word[4:42]
'on'
>>> word[42:]
''Python 中的字符串不能被修改，它们是 immutable 的。因此，向字符串的某个索引位置赋值会产生一个错误:
>>> word[0] = 'J'
Traceback (most recent call last):
File "", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
Traceback (most recent call last):
File "", line 1, in <module>
TypeError: 'str' object does not support item assignment如果需要一个不同的字符串，应当新建一个:
>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'内建函数 len() 返回一个字符串的长度:
>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
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