python 字符串特点

除了数值，Python可以操作字符串，它可以表现在以下几个方面。包含在单引号或双引号：

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

字符串可以写多行。可以用\n表示，下一行是一个合乎逻辑的延续行,最后一个字符用反斜杠：

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

字符串可以被包围在一对三重引号里面：

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

字符串可以被连接在一起，用“+”运算符，重复*：

>>> word = 'Help' + 'A'
>>> word
'HelpA'
>>> '<' + word*5 + '>'
'<HelpAHelpAHelpAHelpAHelpA>'

两个彼此相邻的字符串文字自动连接：

>>> '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

注意：word字符串的内容是： “HelpA”  可以是下标（索引）和C一样，字符串的第一个字符下标（索引）0。可以指定的子串切片标志来表示：两个指数由冒号分隔。

>>> word[4]
'A'
>>> word[0:2]
'He'
>>> word[2:4]
'lp'

切片索引可以使用默认值;前一个索引默认为零，第二个索引默认被切片的字符串的大小。

>>> word[:2]    # The first two characters
'He'
>>> word[2:]    # Everything except the first two characters
'lpA'

和C字符串不同，Python字符串不能改变。想修改指定索引位置的字符串会导致错误：

>>> word[0] = 'x'
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: object doesn't support item assignment
>>> word[:1] = 'Splat'
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: object doesn't support slice assignment

然而，创建一个新的字符串是简单而有效的：

>>> 'x' + word[1:]
'xelpA'
>>> 'Splat' + word[4]
'SplatA'

这里是一个有用的切片操作：[：]+[：]等于。

>>> word[:2] + word[2:]
'HelpA'
>>> word[:3] + word[3:]
'HelpA'

指数可以是负数，从右边开始计数。例如：

>>> word[-1]     # The last character
'A'
>>> word[-2]     # The last-but-one character
'p'
>>> word[-2:]    # The last two characters
'pA'
>>> word[:-2]    # Everything except the last two characters
'Hel'