Caesar Cipher written in Python

caesar_cipher.py

# Constant Values
DEFAULT_SHIFT = 1
MAX_ALPHABET_COUNT = 26
UPPERCASE_A_ASCII_CODE = 65
LOWERCASE_A_ASCII_CODE = 97

def encode(plaintext, shift=DEFAULT_SHIFT):
    if not plaintext.isalpha():
        raise ValueError("Invalid 'plaintext' argument value %s; it is not all alpha." % plaintext)

    ciphertext = ""
    for letter in plaintext:
        if letter.isupper():
            ciphertext += chr((ord(letter) + shift-UPPERCASE_A_ASCII_CODE) % MAX_ALPHABET_COUNT + UPPERCASE_A_ASCII_CODE)
        else:
            ciphertext += chr((ord(letter) + shift-LOWERCASE_A_ASCII_CODE) % MAX_ALPHABET_COUNT + LOWERCASE_A_ASCII_CODE)
    return ciphertext.lower()

def decode(ciphertext, shift=DEFAULT_SHIFT):
    return encode(ciphertext, MAX_ALPHABET_COUNT - shift)

caesar_cipher_test.py

import unittest
import pytest

import caesar_cipher

class CaesarCipherTest(unittest.TestCase):

    def test_encode_letter_with_default_shift_distance(self):
        plaintext = 'a'
        ciphertext = 'b'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext))

    def test_decode_letter_with_default_shift_distance(self):
        plaintext = 'b'
        ciphertext = 'a'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext))

    def test_encode_wraps_z_to_a(self):
        plaintext = 'z'
        ciphertext = 'a'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext))

    def test_decode_wraps_a_to_z(self):
        plaintext = 'a'
        ciphertext = 'z'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext))

    def test_encode_string_with_default_shift_distance(self):
        plaintext = 'wxyzabcd'
        ciphertext = 'xyzabcde'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext))

    def test_decode_string_with_default_shift_distance(self):
        plaintext = 'xyzabcde'
        ciphertext = 'wxyzabcd'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext))

    def test_encode_with_10_shift_distance(self):
        plaintext = 'wxyzabcd'
        ciphertext = 'ghijklmn'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext, 10))

    def test_decode_with_10_shift_distance(self):
        plaintext = 'ghijklmn'
        ciphertext = 'wxyzabcd'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext, 10))

    def test_encode_with_26_shift_distance(self):
        plaintext = 'abcdefghij'
        ciphertext = 'abcdefghij'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext, 26))

    def test_decode_with_26_shift_distance(self):
        plaintext = 'klmnopqrst'
        ciphertext = 'klmnopqrst'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext, 26))

    def test_encode_with_27_shift_distance(self):
        plaintext = 'wxyzabcd'
        ciphertext = 'xyzabcde'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext, 27))

    def test_decode_with_27_shift_distance(self):
        plaintext = 'xyzabcde'
        ciphertext = 'wxyzabcd'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext, 27))

    def test_encode_always_returns_downcase_string(self):
        plaintext = 'AbCdEfGhIj'
        ciphertext = 'bcdefghijk'
        self.assertEqual(ciphertext, caesar_cipher.encode(plaintext))

    def test_decode_always_returns_downcase_string(self):
        plaintext = 'BcDeFgHiJk'
        ciphertext = 'abcdefghij'
        self.assertEqual(ciphertext, caesar_cipher.decode(plaintext))

    # pylint: disable=no-self-use
    def test_encode_raise_error_if_non_letter_characters(self):
        plaintext = 'az4'
        with pytest.raises(Exception):
            caesar_cipher.encode(plaintext)

    # pylint: disable=no-self-use
    def test_decode_raise_error_if_non_letter_characters(self):
        plaintext = 'az4'
        with pytest.raises(Exception):
            caesar_cipher.decode(plaintext)

if __name__ == '__main__':
    unittest.main()