dawkins weasel program

# This is a weasel constructor.
# Cf. The Blind Watchmaker, Richard Dawkins, Chapter 3

from random import choice

##
# Weasel constructor

def weasel(target='METHINKS IT IS LIKE A WEASEL',
           characters='default', mode=0):
    """mode 0: single step selection; mode 1: cumulative selection"""
    if characters == 'default':
        characters = [chr(ascii_code) for ascii_code in range(65,91)] # 'A'-'Z'
        characters += ' '
    working_string = ''
    tries = 0
    if mode == 0: # = single step selection mode
        while working_string != target:
        # run until target found (potentially a very long time)
            working_string = ''
            for position in range(len(target)):
                working_string += choice(characters)
                # fill working_string with random characters to len of target
            print working_string
            tries += 1
    if mode == 1: # cumulative selection mode
        for character in range(len(target)):
            working_string += choice(characters)
        tries += 1
        while working_string != target:
            for index_position in range(len(target)):
                if working_string[index_position] == target[index_position]:
                    pass
                else:
                    working_string = working_string[0:index_position] + choice(characters) + working_string[index_position+1:]
            print working_string
            tries += 1
    print "Total tries:",tries
    return working_string, tries

weasel(target='METHINKS IT IS LIKE A WEASEL', mode=1)