3.1.2. 字符串

除了数字,Python 也可以操作字符串。字符串有多种形式,可以使用单引号('…'),双引号("…")都可以获得同样的结果 2。反斜杠 \ 可以用来转义:

  1. >>> 'spam eggs' # single quotes
  2. 'spam eggs'
  3. >>> 'doesn\'t' # use \' to escape the single quote...
  4. "doesn't"
  5. >>> "doesn't" # ...or use double quotes instead
  6. "doesn't"
  7. >>> '"Yes," they said.'
  8. '"Yes," they said.'
  9. >>> "\"Yes,\" they said."
  10. '"Yes," they said.'
  11. >>> '"Isn\'t," they said.'
  12. '"Isn\'t," they said.'

在交互式解释器中,输出的字符串外面会加上引号,特殊字符会使用反斜杠来转义。 虽然有时这看起来会与输入不一样(外面所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,该字符串外将加双引号来表示,否则就加单引号。 print() 函数会生成可读性更强的输出,即略去两边的引号,并且打印出经过转义的特殊字符:

  1. >>> '"Isn\'t," they said.'
  2. '"Isn\'t," they said.'
  3. >>> print('"Isn\'t," they said.')
  4. "Isn't," they said.
  5. >>> s = 'First line.\nSecond line.' # \n means newline
  6. >>> s # without print(), \n is included in the output
  7. 'First line.\nSecond line.'
  8. >>> print(s) # with print(), \n produces a new line
  9. First line.
  10. Second line.

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

  1. >>> print('C:\some\name') # here \n means newline!
  2. C:\some
  3. ame
  4. >>> print(r'C:\some\name') # note the r before the quote
  5. C:\some\name

字符串字面值可以跨行连续输入。一种方式是用三重引号:"""…"""'''…'''。字符串中的回车换行会自动包含到字符串中,如果不想包含,在行尾添加一个 \ 即可。如下例:

  1. print("""\
  2. Usage: thingy [OPTIONS]
  3. -h Display this usage message
  4. -H hostname Hostname to connect to
  5. """)

将产生如下输出(注意最开始的换行没有包括进来):

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

字符串可以用 + 进行连接(粘到一起),也可以用 * 进行重复:

  1. >>> # 3 times 'un', followed by 'ium'
  2. >>> 3 * 'un' + 'ium'
  3. 'unununium'

相邻的两个或多个 字符串字面值 (引号引起来的字符)将会自动连接到一起.

  1. >>> 'Py' 'thon'
  2. 'Python'

把很长的字符串拆开分别输入的时候尤其有用:

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

只能对两个字面值这样操作,变量或表达式不行:

  1. >>> prefix = 'Py'
  2. >>> prefix 'thon' # can't concatenate a variable and a string literal
  3. File "<stdin>", line 1
  4. prefix 'thon'
  5. ^
  6. SyntaxError: invalid syntax
  7. >>> ('un' * 3) 'ium'
  8. File "<stdin>", line 1
  9. ('un' * 3) 'ium'
  10. ^
  11. SyntaxError: invalid syntax

如果你想连接变量,或者连接变量和字面值,可以用 + 号:

  1. >>> prefix + 'thon'
  2. 'Python'

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:

  1. >>> word = 'Python'
  2. >>> word[0] # character in position 0
  3. 'P'
  4. >>> word[5] # character in position 5
  5. 'n'

索引也可以用负数,这种会从右边开始数:

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

注意 -0 和 0 是一样的,所以负数索引从 -1 开始。

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:

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

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

  1. >>> word[:2] + word[2:]
  2. 'Python'
  3. >>> word[:4] + word[4:]
  4. 'Python'

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:

  1. >>> word[:2] # character from the beginning to position 2 (excluded)
  2. 'Py'
  3. >>> word[4:] # characters from position 4 (included) to the end
  4. 'on'
  5. >>> word[-2:] # characters from the second-last (included) to the end
  6. '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:

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

第一行数标注了字符串 0…6 的索引的位置,第二行标注了对应的负的索引。那么从 ij 的切片就包括了标有 ij 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。

试图使用过大的索引会产生一个错误:

  1. >>> word[42] # the word only has 6 characters
  2. Traceback (most recent call last):
  3. File "<stdin>", line 1, in <module>
  4. IndexError: string index out of range

但是,切片中的越界索引会被自动处理:

  1. >>> word[4:42]
  2. 'on'
  3. >>> word[42:]
  4. ''

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:

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

如果需要一个不同的字符串,应当新建一个:

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

内建函数 len() 返回一个字符串的长度:

  1. >>> s = 'supercalifragilisticexpialidocious'
  2. >>> len(s)
  3. 34

参见