字符串
相比数值，Python 也提供了可以通过几种不同方式表示的字符串。它们可以用单引号 ('...') 或双引号 ("...") 标识[2]。\ 可以用来转义引号:
>>> '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," he said.'
'"Yes," he said.'
>>> "\"Yes,\" he said."
'"Yes," he said.'
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
在交互式解释器中，输出的字符串会用引号引起来，特殊字符会用反斜杠 (\) 转义。虽然可能和输入看上去不太一样，但是两个字符串是相等的。如果字符串中只有单引号而没有双引号，就用双引号引用，否则用单引号引用。再强调一下，print 语句可以生成可读性更好的输出:
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
>>> print '"Isn\'t," she said.'
"Isn't," she 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.
如果你前面带有 \ 的字符被当作特殊字符，你可以使用原始字符串，方法是在第一个引号前面加上一个 r:
>>> 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
字符串文本能够分成多行。一种方法是使用三引号："""...""" 或者'''...'''。行尾换行符会被自动包含到字符串中，但是可以在行尾加上\ 来避免这个行为。下面的示例：可以使用反斜杠为行结尾的连续字符串，它表示下一行在逻辑上是本行的后续内容:
print """\
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
"""
将生成以下输出（注意，没有开始的第一行）:
Usage: thingy [OPTIONS]
-h Display this usage message
-H hostname Hostname to connect to
字符串可以由 + 操作符连接(粘到一起)，可以由* 表示重复:
>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'
相邻的两个字符串文本自动连接在一起。:
>>> 'Py' 'thon'
'Python'
它只用于两个字符串文本，不能用于字符串表达式:
>>> prefix = 'Py'
>>> prefix 'thon' # can't concatenate a variable and a string literal
...
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
...
SyntaxError: invalid syntax
如果你想连接多个变量或者连接一个变量和一个字符串文本，使用 +:
>>> prefix + 'thon'
'Python'
这个功能在你想切分很长的字符串的时候特别有用:
>>> text = ('Put several strings within parentheses '
'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'
字符串也可以被截取(检索)。类似于 C ，字符串的第一个字符索引为 0 。Python没有单独的字符类型；一个字符就是一个简单的长度为1的字符串。:
>>> word = 'Python'
>>> word[0] # character in position 0
'P'
>>> word[5] # character in position 5
'n'
索引也可以是负数，这将导致从右边开始计算。例如:
>>> word[-1] # last character
'n'
>>> word[-2] # second-last character
'o'
>>> word[-6]
'P'
请注意 -0 实际上就是 0，所以它不会导致从右边开始计算。
除了索引，还支持 切片。索引用于获得单个字符，切片 让你获得一个子字符串:
>>> 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，包含起始的字符，不包含末尾的字符。这使得 s[:i] + s[i:] 永远等于s:
>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'
切片的索引有非常有用的默认值；省略的第一个索引默认为零，省略的第二个索引默认为切片的字符串的大小。:
>>> 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'
有个办法可以很容易地记住切片的工作方式：切片时的索引是在两个字符 之间 。左边第一个字符的索引为 0，而长度为n 的字符串其最后一个字符的右界索引为 n。例如:
+---+---+---+---+---+---+
| P | y | t | h | o | n |
+---+---+---+---+---+---+
0 1 2 3 4 5 6
-6 -5 -4 -3 -2 -1
文本中的第一行数字给出字符串中的索引点 0...6。第二行给出相应的负索引。切片是从 i 到j 两个数值标示的边界之间的所有字符。
对于非负索引，如果上下都在边界内，切片长度就是两个索引之差。例如，word[1:3] 是 2 。
试图使用太大的索引会导致错误:
>>> 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
Python 能够优雅地处理那些没有意义的切片索引：一个过大的索引值(即下标值大于字符串实际长度)将被字符串实际长度所代替，当上边界比下边界大时(即切片左值大于右值)就返回空字符串:
>>> word[4:42]
'on'
>>> word[42:]
''
Python字符串不可以被更改 — 它们是 不可变 的。因此，赋值给字符串索引的位置会导致错误:
>>> word[0] = 'J'
...
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
...
TypeError: 'str' object does not support item assignment
如果你需要一个不同的字符串，你应该创建一个新的:
>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'
内置函数 len() 返回字符串长度:
>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34
