#yyds干货盘点#python字符串

除了数字，Python 还可以操作字符串。字符串有多种表现形式，用单引号（'……'）或双引号（"……"）标注的结果相同 ​​2​​。反斜杠 \ 用于转义：

>>> '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," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'

交互式解释器会为输出的字符串加注引号，特殊字符使用反斜杠转义。虽然，有时输出的字符串看起来与输入的字符串不一样（外加的引号可能会改变），但两个字符串是相同的。如果字符串中有单引号而没有双引号，该字符串外将加注双引号，反之，则加注单引号。​​print()​ 函数输出的内容更简洁易读，它会省略两边的引号，并输出转义后的特殊字符：

>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'
>>> print('"Isn\'t," they said.')
"Isn't," they 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.

如果不希望前置 \ 的字符转义成特殊字符，可以使用 原始字符串，在引号前添加 r 即可：

>>> 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

字符串字面值可以包含多行。 一种实现方式是使用三重引号："""...""" 或 '''...'''。 字符串中将自动包括行结束符，但也可以在换行的地方添加一个 \ 来避免此情况。 参见以下示例：

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

输出如下（请注意开始的换行符没有被包括在内）：

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

字符串可以用 + 合并（粘到一起），也可以用 * 重复：

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

相邻的两个或多个 字符串字面值 （引号标注的字符）会自动合并：

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

拆分长字符串时，这个功能特别实用：

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

这项功能只能用于两个字面值，不能用于变量或表达式：

>>> prefix = 'Py'
>>> prefix 'thon' # can't concatenate a variable and a string literal
File "<stdin>", line 1
prefix 'thon'
^
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
File "<stdin>", line 1
('un' * 3) 'ium'
^
SyntaxError: invalid syntax

合并多个变量，或合并变量与字面值，要用 +：

>>> prefix + 'thon'
'Python'

字符串支持 索引 （下标访问），第一个字符的索引是 0。单字符没有专用的类型，就是长度为一的字符串：

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

索引还支持负数，用负数索引时，从右边开始计数：

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

注意，-0 和 0 一样，因此，负数索引从 -1 开始。

除了索引，字符串还支持 切片。索引可以提取单个字符，切片 则提取子字符串：

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

切片索引的默认值很有用；省略开始索引时，默认值为 0，省略结束索引时，默认为到字符串的结尾：

>>> 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'

注意，输出结果包含切片开始，但不包含切片结束。因此，s[:i] + s[i:] 总是等于 s：

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

还可以这样理解切片，索引指向的是字符 之间 ，第一个字符的左侧标为 0，最后一个字符的右侧标为 n ，n 是字符串长度。例如：

+---+---+---+---+---+---+
| P | y | t | h | o | n |
+---+---+---+---+---+---+
0 1 2 3 4 5 6
-6 -5 -4 -3 -2 -1

第一行数字是字符串中索引 0...6 的位置，第二行数字是对应的负数索引位置。i 到 j 的切片由 i 和 j 之间所有对应的字符组成。

对于使用非负索引的切片，如果两个索引都不越界，切片长度就是起止索引之差。例如， word[1:3] 的长度是 2。

索引越界会报错：

>>> 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

但是，切片会自动处理越界索引：

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

Python 字符串不能修改，是 immutable 的。因此，为字符串中某个索引位置赋值会报错：

>>> word[0] = 'J'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

要生成不同的字符串，应新建一个字符串：

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

内置函数 ​​len()​​ 返回字符串的长度：

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