python 字串特點

除了數值，Python可以操作字串，它可以表現在以下幾個方面。包含在單引號或雙引號：

>>> '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.'

字串可以寫多行。可以用\n表示，下一行是一個合乎邏輯的延續行,最後一個字元用反斜線：

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

字串可以被包圍在一對三重引號裡面：

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

字串可以被串連在一起，用“+”運算子，重複*：

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

兩個彼此相鄰的字串文字自動連接：

>>> '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字串的內容是： “HelpA”  可以是下標（索引）和C一樣，字串的第一個字元下標（索引）0。可以指定的子串切片標誌來表示：兩個指數由冒號分隔。

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

切片索引可以使用預設值;前一個索引預設為零，第二個索引預設被切片的字串的大小。

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

和C字串不同，Python字串不能改變。想修改指定索引位置的字串會導致錯誤：

>>> 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，建立一個新的字串是簡單而有效：

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

這裡是一個有用的切片操作：[：]+[：]等於。

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

指數可以是負數，從右邊開始計數。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'