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 True if letter in letters else False
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?
Variables can be patched as follows:
from mock import patch
@patch('module.variable', new_value)
For example:
import alphabet
from mock import patch
@patch('alphabet.letters', ['a', 'b', 'c'])
class TestAlphabet():
def test_length_letters(self):
assert 3 == alphabet.length_letters()
def test_contains_letter(self):
assert alphabet.contains_letter('a')
Try this:
import unittests
import alphabet
from unittest import mock
class TestAlphabet(unittest.TestCase):
def setUp(self):
self.mock_letters = mock.patch.object(
alphabet, 'letters', return_value=['a', 'b', 'c']
)
def test_length_letters(self):
with self.mock_letters:
self.assertEqual(3, alphabet.length_letters())
def test_contains_letter(self):
with self.mock_letters:
self.assertTrue(alphabet.contains_letter('a'))
You need to apply the mock while the individual tests are actually running, not just in setUp()
. We can create the mock in setUp()
, and apply it later with a with ...
Context Manager.