double.py

"""
Create a function that accepts a string. The function should return a string, with each character doubled.
E.g. MMaarrttiinn
"""


def double_characters(word):
    if not word:
        raise ValueError('You gave an empty list.')
    double = ""
    for character in word:
        double += character * 2
    return double


if __name__ == "__main__":
    word = "Martin"
    print(double_characters(word))

test_double.py

import unittest

from double import double_characters


class TestDouble(unittest.TestCase):
    def test_double_characters_double_characters(self):
        self.assertEqual(double_characters('Martin'), 'MMaarrttiinn')
        self.assertEqual(double_characters('Tree'), 'TTrreeee')
        
        
    def test_double_charcaters_fails_on_empty_string(self):
        self.assertRaises(ValueError, double_characters, '')