Python Cookbook

Filtering a String for a Set of Characters

Credit: Jürgen Hermann, Nick Perkins

Problem

Given a set of characters to keep, you need to build a filtering functor (a function-like, callable object). The specific functor you need to build is one that, applied to any string s, returns a copy of s that contains only characters in the set.

Solution

The string.maketrans function and translate method of string objects are fast and handy for all tasks of this ilk:

import string

# Make a reusable string of all characters
_allchars = string.maketrans('', '')

def makefilter(keep):
    """ Return a functor that takes a string and returns a partial copy of that
        string consisting of only the characters in 'keep'.
    """
    # Make a string of all characters that are not in 'keep'
    delchars = _allchars.translate(_allchars, keep)

    # Return the functor, binding the two strings as default args
    return lambda s, a=_allchars, d=delchars: s.translate(a, d)

def canonicform(keep):
    """ Given a string, considered as a set of characters, return the
        string's characters as a canonic-form string: alphabetized
        and without duplicates.
    """
    return makefilter(keep)(_allchars)

if _ _name_ _ == '_ _main_ _':
    identifier = makefilter(string.letters + string.digits + '_')
    print identifier(_allchars)

Discussion

The key to understanding this recipe lies in the definitions of the translate and maketrans functions in the string module. translate takes a string and replaces each character in it with the corresponding character in the translation ...