Python——string之maketrans，translate函數

        先來看下關於這兩個函數的官方定義：

        string.maketrans(from, to)：Return a translation table suitable for passing to translate(), that will map each character in from into the character at the same position in to; from and to must have the same length.

        string.translate(s, table[, deletechars])：Delete all characters from s that are in deletechars (if present), and then translate the characters using table, which must be a 256-character string giving the translation for
each character value, indexed by its ordinal. If table is None, then only the character deletion step is performed.

        下面的代碼是對這兩個函數進行的封裝：

#!/usr/bin/env python# -*- coding:utf-8 -*-import stringdef translator(frm='', to='', delete='', keep=None):    if len(to) == 1:        to = to * len(frm)        trans = string.maketrans(frm, to)    if keep is not None:        trans_all = string.maketrans('', '')        #keep.translate(trans_all, delete)，從要保留的字元中剔除要刪除的字元        #trans_all.translate(trans_all, keep.translate(trans_all, delete))，從翻譯表中刪除要保留的字元，即取保留字元的補集        delete = trans_all.translate(trans_all, keep.translate(trans_all, delete))            def translate(s):        return s.translate(trans, delete)            return translateif __name__ == '__main__':    #result:12345678    digits_only = translator(keep=string.digits)    print digits_only('Eric chen: 1234-5678')        #result:Eric chen: -    no_digits = translator(delete=string.digits)    print no_digits('Eric chen: 1234-5678')        #result:Eric chen: ****-****    digits_to_hash = translator(frm=string.digits, to='*')    print digits_to_hash('Eric chen: 1234-5678')

        當以string.maketrans('', '')方法調用maketrans時，翻譯表正好是有256個字元的字串t。翻譯表產生的字串（忽略不可列印字元）為“!"#$%&'()*+,-./:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~”，本質上與ASCII表相對應。

        其實調用maketrans函數時，已經完成了轉換。例如string.maketrans('ABCD', 'abcd'),調用完成後，翻譯表產生的包含256個字元的字串（忽略不可列印字元）為“!"#$%&'()*+,-./0123456789:;<=>?@abcdEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~”，該翻譯表中的原“ABCD”的位置已被“abcd”替換。

        當你把t作為第一個參數傳入translate方法時，原字串中的每一個字元c，在處理完成後都會被翻譯成字元t[ord(c)]。

        For Unicode objects, the translate() method does not accept the optional deletechars argument. Instead, it returns a copy of the s where all characters have been mapped through the given translation table which must be
a mapping of Unicode ordinals to Unicode ordinals, Unicode strings or None. Unmapped characters are left untouched. Characters mapped to None are deleted.

        下面的代碼是對unicode字串進行過濾：

import setsclass Keeper(object):    def __init__(self, keep):        self.keep = sets.Set(map(ord, keep))        def __getitem__(self, n):        if n not in self.keep:            return None        return unichr(n)        def __call__(self, s):        return unicode(s).translate(self)makeFilter = Keeperif __name__ == '__main__':    #result:人民    just_people = makeFilter(u'人民')    print just_people(u'中華人民共和國成立了')    #刪除unicode字元    #result:中華共和國成立了!    translate_table = dict((ord(char), None) for char in u'人民')    print unicode(u'中華人民共和國成立了!').translate(translate_table)        #替換unicode字元    #result:中華**共和國成立了!    translate_table = dict((ord(char), u'*') for char in u'人民')    print unicode(u'中華人民共和國成立了!').translate(translate_table)

        Unicode字串的translate方法只需要一個參數：一個序列或映射，並且根據字串中的每個字元的碼值進行索引。碼值不是一個映射的鍵（或者序列的索引值）的字元會被直接複製，不做改變。與每個字元碼對應的值必須是一個unicode字串（該字元的替換物）或者None（這意味著該字元需要被刪除）。通常我們使用dict或list作為unicode字串的translate方法的參數，來翻譯或刪除某些字元。