Function permutations

python/Chapter 9/Question9_05/permutations.py:15–34

Return a generator of all the permutations of the string.

(str_)

import unittest

def permutations(str_):
    """ Return a generator of all the permutations of the string. """

    # A string such as 'a' has itself as its only permutation, while for 'ab'
    # the permutations are 'ab' and 'ba'. For 'abc', a three-character string,
    # there are six permutations: 'abc', 'acb', 'bac', 'bca', 'cab', 'cba'. Two
    # of these permutations, 'abc' and 'acb', start with 'a', and what follows
    # this first character is, respectively, 'bc' and 'cb': the two possible
    # permutations of 'cb'. This allows us to see the pattern here: for each
    # letter in the string we need to concatenate to it the sub-permutations
    # that can be generated when the letter is removed from the string.

    if len(str_) == 1:
        yield str_

    for index, letter in enumerate(str_):
        # The string without the index-th letter
        without = str_[:index] + str_[index + 1:]
        for sub_permutation in permutations(without):
            yield letter + sub_permutation

class PermutationsTest(unittest.TestCase):