20201228-2 字符串

4 Strings
4-1
>>> '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.'

4-2
>>> '"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.

4-3
>>> 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

4-4
String literals can span multiple lines. 
One way is using triple-quotes: """...""" or '''...'''. 
End of lines are automatically included in the string, 
but it’s possible to prevent this by adding a \ at the end of the line. 
The following example:

print("""\
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to
""")
produces the following output (note that the initial newline is not included):

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

4-5
Strings can be concatenated (glued together) with the + operator, and repeated with *:
>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'

4-6
Two or more string literals (i.e. the ones enclosed between quotes) next to each other 
are automatically concatenated.
>>> 'Py' 'thon'
'Python'

4-7
This feature is particularly useful when you want to break long strings:
>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'

4-8
This only works with two literals though, not with variables or expressions:
>>> 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

4-9
If you want to concatenate variables or a variable and a literal, use +:
>>> prefix + 'thon'
'Python

4-10
字符串是可以被 索引 （下标访问）的，第一个字符索引是 0。单个字符并没有特殊的类型，只是一个长度为一的字符串:
>>> word = 'Python'
>>> word[0]  # character in position 0
'P'
>>> word[5]  # character in position 5
'n'

4-11
索引也可以用负数，这种会从右边开始数:
>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]
'P'
注意 -0 和 0 是一样的，所以负数索引从 -1 开始。

4-12
除了索引，字符串还支持 切片。索引可以得到单个字符，而 切片 可以获取子字符串:
>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

4-13
注意切片的开始总是被包括在结果中，而结束不被包括。这使得 s[:i] + s[i:] 总是等于 s
>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

4-14
切片的索引有默认值；省略开始索引时默认为0，省略结束索引时默认为到字符串的结束:
>>> 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'

您也可以这么理解切片：将索引视作指向字符 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 ，其中 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

第一行数标注了字符串非负的索引的位置，
第二行标注了对应的负的索引。
那么从 i 到 j 的切片就包括了标有 i 和 j 的位置之间的所有字符。

对于使用非负索引的切片，如果索引不越界，那么得到的切片长度就是起止索引之差。例如， word[1:3] 的长度为2.

4-15
使用过大的索引会产生一个错误:
>>> 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

4-16
但是，切片中的越界索引会被自动处理:
>>> word[4:42]
'on'
>>> word[42:]
''

4-17
Python 中的字符串不能被修改，它们是 immutable 的。因此，向字符串的某个索引位置赋值会产生一个错误:
>>> 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

4-18
如果需要一个不同的字符串，应当新建一个:
>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'

4-19
内建函数 len() 返回一个字符串的长度:
>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34