python 基础——字符串

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python 中对于字符串的处理，也是相当厉害的

不仅支持传统编程语言中的双引号模式，也支持 shell 中的单引号模式，甚至还可以用三引号一次搞定多行字符串！

一对单引号的模式

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>>> 'spam eggs'
'spam eggs'
>>> 'doesn\'t'
"doesn't"
>>> '"Yes," he said.'
'"Yes,"he said.'
>>> '"Isn\'t,"she said.'
'"Isn\'t,"she said.'

一对双引号的模式

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>>> "\"Yes,\"he said."
'"Yes," he said.'
>>> hello = "This is a rather long string containing\n\
... several lines of text just as you would do in C.\n\
...     Note that whitespace at the beginning of the line is\
...  significant."
>>> print hello
This is a rather long string containing
several lines of text just as you would do in C.
    Note that whitespace at the beginning of the line is significant.

一对三引号的模式

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>>> print """\
... Usage: thingy [OPTIONS]
...      -h                        Display this usage message
...      -H hostname               Hostname to connect to
... """
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to

总结：

如果字符串中只包含单引号，不包含双引号，那就可以用一对双引号给包起来

如果字符串中之包含双引号，不包含单引号，那就可以用一对单引号给包起来

当需要使用多行的字符串时，可以使用一对三引号，被三引号给包起来的所有内容不进行转义，即所有转义字符无效

同时，还可以使用 “原始” 字符串，与三引号具有同样的效果，只需要在引号前面加个“r”

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>>> hello = r"This is a rather long string containing\n\
... several lines of text much as you would do in C."
>>> print(hello)
This is a rather long string containing\n\
several lines of text much as you would do in C.

字符串还可以任意拼接和重复

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>>> '<' + 'niko'*5 + '>'
'< niko  niko  niko  niko  niko >'

相邻的两个引号内的字符串即使不用加号也会自动拼接在一起，但在调用方法时不适用

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>>> 'str' 'ing'                   #  <-  This is ok
'string'
>>> 'str'.strip() + 'ing'   #  <-  This is ok
'string'
>>> 'str'.strip() 'ing'     #  <-  This is invalid
  File "<stdin>", line 1, in ?
    'str'.strip() 'ing'
                      ^
SyntaxError: invalid syntax

可以使用冒号来取任意长度的连续字符串

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>>> word = 'str' 'ing'
>>> word[4]     # 取 4
'n'
>>> word[2:]    #2 到最后
'ring'
>>> word[:2]    # 最前到 2
'st'
>>> word[0:2]   # 取 0-2
'st'

python 字符串定义后为常量，不可任意修改其中的某个字符

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>>> word[2] = 'k'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

索引也可以为负值，即自右向左算

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>>> word[-2:]
'ng'
>>> word[:-2]
'stri'

-0 其实就是 0，官方给出的示例索引表如下

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 +---+---+---+---+---+
 | H | e | l | p | A |
 +---+---+---+---+---+
 0   1   2   3   4   5
-5  -4  -3  -2  -1

字符串求长度函数为 len()

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>>> s = 'jreiouwtndouijfnbvioftretjk'
>>> len(s)
27