Python-Cookbook-1.09 简化 translate 方法

2016-05-25

string.maketrans

Help on built-in function maketrans in module strop:
maketrans(...)
    maketrans(frm, to) -> string
    Return a translation table (a string of 256 bytes long)
    suitable for use in string.translate.  The strings frm and to
    must be of the same length.
(END)

生成一个供string.translate使用的 ASCII 表，其中frm中的所有字符都依序被替换成to中字符：

>>> maketrans('abc', 'fed')
'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`feddefghijklmnopqrstuvwxyz{|}~\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff'
>>>

string.translate

Help on function translate in module string:
translate(s, table, deletions='')
    translate(s,table [,deletions]) -> string
    Return a copy of the string s, where all characters occurring
    in the optional argument deletions are removed, and the
    remaining characters have been mapped through the given
    translation table, which must be a string of length 256.  The
    deletions argument is not allowed for Unicode strings.
(END)

(也可以s.translate(table, deletions='')为格式）

以maketrans生成的映射表为基准进行字符转换：

>>> translate('abcdef', a)
'feddef'
>>> translate('abcdef', a, 'd')
'fedef'
>>> translate('abcdef', a, 'dd')
'fedef'
>>> translate('abcdef', a, 'de')
'fedf'
>>> translate('abcdef', a, 'ade')
'edf'
>>>

自建一个返回闭包的工厂函数 translator

import string
def translator(frm='', to='', delete='', keep=None):
    if len(to) == 1:
        to = to * len(frm)
    trans = string.maketrans(frm, to)
    if keep is not None:
        allchars = string.maketrans('', '')
        delete = allchars.translate(allchars, keep.translate(allchars, delete))
    def translate(s):
        return s.translate(trans, delete)
        
    return translate
if __name__ == '__main__':
    digits_only = translator(keep=string.digits)
    print digits_only('qwedwefaf24215')
    no_digits = translator(delete=string.digits)
    print no_digits('qwedwefaf24215')