[Python] Mocking a global variable

I've been trying to implement some unit tests for a module. An example module named alphabet.py is as follows:

import database

def length_letters():
return len(letters)

def contains_letter(letter):
return letter in letters


letters = database.get('letters') # returns a list of letters


I'd like to mock the response from a database with some values of my choice, but the code below doesn't seem to work.

import unittests
import alphabet
from unittest.mock import patch


class TestAlphabet(unittest.TestCase):
@patch('alphabet.letters')
def setUp(self, mock_letters):
mock_letters.return_value = ['a', 'b', 'c']

def test_length_letters(self):
self.assertEqual(3, alphabet.length_letters())

def test_contains_letter(self):
self.assertTrue(alphabet.contains_letter('a'))


I have seen many examples in which 'patch' is applied to methods and classes, but not to variables. I prefer not to patch the method database.get because I may use it again with different parameters later on, so I would need a different response.

What am I doing wrong here?

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