Mocking a Global Variable

Mocking a global variable

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.

How to mock an uninitialized global variable?

I used the setUp and tearDown methods on the unittest.TestCase class to solve this.

With my edit above about it actually being a config object the code looks like this:

class Config:
    def __init__(self, filename: str):
        "Load config file and set self.stuff"
        self.stuff = _code_to_parse(filename)
config: Config
def do_stuff() -> str:
    return "a" if config.stuff else "b"

And the test creates a mocked config object before the tests in order to mock some attribute on the config object during the test. In the teardown I also delete the object again so that nothing will spill to the next test case.

The actual mocking of the attributes I need is then done in a with block in the actual test as it is (a) cleaner and (b) I can just mock what I need in contrast to mocking all attributes on the config object in the setUp method.

import unittest, unittest.mock, mod
class MockUninitializedGlobal(unittest.TestCase):
    def setUp(self):
        mod.config = unittest.mock.Mock(spec=mod.Config)
    def tearDown(self):
        del mod.config
    def test_it(self):
        with unittest.mock.patch("mod.config.stuff", True):
            actual = mod.do_stuff()
        expected = "a"
        self.assertEqual(expected, actual)

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