py3学习笔记2（字符串）

　　除了数字计算，python还可以通过多种方式操作字符串

　　他们可以被封装在单引号（'...'）或者（"..."）双引号中，使用这两种方式都可以得到相同的结果。（在单引号中，'\n'等将不像在双引号中和C语言等其他语言的字符串定义中显示回车，而是显示其本身，只有显示单引号时使用'\''）

　　使用'\'可以摆脱引号的引用关系与字符意义。

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

　　使用print()函数将获得一个可读性更好的字符串

>>> '"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.

　　如果你需要将'\'以及其后的字符显示字符意义，可以在字符串前加'r'转义成行字符串

>>> 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

 　　有些字符串需要占用许多行，这时可以使用"""..."""或者'''...'''行尾将自动地包含在字符串中，也可以加入额外的'\'来确保这一点。

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

　　以上代码将打印

Usage: thingy [OPTIONS]
    -h Display this usage message
    -H hostname Hostname to connect to

 

　　另外，字符串可以使用'+'与'*'进行操作

>>> # 3 times 'un', followed by 'ium'
>>> 3*'un' + 'ium'
'unununium'

　　两个或多个相邻的字符串将自动组合在一起

>>> 'Py' 'thon'
'Python'

　　但无法使用变量对其进行以上操作

>>> prefix = 'Py'
>>> prefix 'thon' # can't concatenate a variable and a string literal
    ...
SyntaxError: invalid syntax
>>> ('un'*3) 'ium'
    ...
SyntaxError: invalid syntax

　　如果你渴望使用变量对字符串进行连接，可以使用'+'

>>> prefix + 'thon'
'Python'

 　　这个特性可以用于当你想分割一个非常长的句子时

>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'    

　　你也可以使用类似于其他语言中队数组的索引(index)操作来操作字符串

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

　　负数将从最后开始倒着索引

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

　　当然，-0和0是一样的，负数索引从-1开始。

 

　　除了索引，python字符串还支持切片（slice），截取字符串中的一部分

>>> word[0:2] # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5] # characters from position 2 (included) to 5 (excluded)
'tho'

　　如下形式的两个切片形式将组成一个完整的字符串

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

　　当冒号左侧没有数字时默认将从头开始，右侧没有数字时将读到末尾

>>> 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'

　　

　　你可以用以下这种方法简介地记住切片的位置

　　

　　使用index方法访问超过数组长度的元素将返回错误

>>> 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

　　而使用slice方法将不受限制，超出时将默认为字符串末尾，没有内容时将返回空字符串

>>> word[4:42]
'on'
>>> word[42:]
''

 

　　注意，在python中，字符串一旦被定义将无法被改变，以下试图改变字符串内容的行为都将报错

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

　　如果你需要一个不一样的字符串，只能创建一个新的

>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'

　　

　　另外，你也可以使用len()函数来获取一个字符串的长度

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