Permutations of a String

permutations.py

import unittest


# Code

def get_permutations(input_str):
    input_len = len(input_str)
    results = []

    if input_len == 0:
        results.append('')
    elif input_len == 1:
        # Return the original
        results.append(input_str)
    else:
        for index in range(input_len):
            # For each character, add to results the character followed by the permutations of the rest
            input_copy = list(input_str)
            char = input_copy[index]
            del input_copy[index]
            sub_permutations = get_permutations(''.join(input_copy))
            results += [f'{char}{permutation}' for permutation in sub_permutations]

    return results



# Tests

class PermutationTests(unittest.TestCase):
    def test_empty_str(self):
        results = get_permutations('')
        expected = ['']
        self.assertEqual(results, expected)

    def test_one_char(self):
        results = get_permutations('a')
        expected = ['a']
        self.assertEqual(results, expected)

    def test_two_chars(self):
        results = get_permutations('ab')
        expected = ['ab', 'ba']
        self.assertListEqual(results, expected)

    def test_three_chars(self):
        results = get_permutations('abc')
        expected = ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
        self.assertListEqual(results, expected)

    def test_four_chars(self):
        results = get_permutations('abcd')
        expected = [
            'abcd', 'abdc', 'acbd', 'acdb', 'adbc', 'adcb',
            'bacd', 'badc', 'bcad', 'bcda', 'bdac', 'bdca',
            'cabd', 'cadb', 'cbad', 'cbda', 'cdab', 'cdba',
            'dabc', 'dacb', 'dbac', 'dbca', 'dcab', 'dcba'
         ]
        # For each additional character, the results include every permutation of the remaining characters following
        self.assertListEqual(results, expected)

    def test_five_chars(self):
        results = get_permutations('abcde')
        expected = [
            'abcde', 'abced', 'abdce', 'abdec', 'abecd', 'abedc', 'acbde', 'acbed', 'acdbe', 'acdeb', 'acebd', 'acedb',
            'adbce', 'adbec', 'adcbe', 'adceb', 'adebc', 'adecb', 'aebcd', 'aebdc', 'aecbd', 'aecdb', 'aedbc', 'aedcb',
            'bacde', 'baced', 'badce', 'badec', 'baecd', 'baedc', 'bcade', 'bcaed', 'bcdae', 'bcdea', 'bcead', 'bceda',
            'bdace', 'bdaec', 'bdcae', 'bdcea', 'bdeac', 'bdeca', 'beacd', 'beadc', 'becad', 'becda', 'bedac', 'bedca',
            'cabde', 'cabed', 'cadbe', 'cadeb', 'caebd', 'caedb', 'cbade', 'cbaed', 'cbdae', 'cbdea', 'cbead', 'cbeda',
            'cdabe', 'cdaeb', 'cdbae', 'cdbea', 'cdeab', 'cdeba', 'ceabd', 'ceadb', 'cebad', 'cebda', 'cedab', 'cedba',
            'dabce', 'dabec', 'dacbe', 'daceb', 'daebc', 'daecb', 'dbace', 'dbaec', 'dbcae', 'dbcea', 'dbeac', 'dbeca',
            'dcabe', 'dcaeb', 'dcbae', 'dcbea', 'dceab', 'dceba', 'deabc', 'deacb', 'debac', 'debca', 'decab', 'decba',
            'eabcd', 'eabdc', 'eacbd', 'eacdb', 'eadbc', 'eadcb', 'ebacd', 'ebadc', 'ebcad', 'ebcda', 'ebdac', 'ebdca',
            'ecabd', 'ecadb', 'ecbad', 'ecbda', 'ecdab', 'ecdba', 'edabc', 'edacb', 'edbac', 'edbca', 'edcab', 'edcba'
        ]
        self.assertListEqual(results, expected)