python学习 字符串

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 lineFirst 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:\someame>>> print r'C:\some\name'  # note the r before the quoteC:\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 charactersTraceback (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