How to add an equality function that is used in collection comparisons in Python unittest

Question

I want to use a custom comparison function when I am unit testing a class. unittest has addTypeEqualityFunc but that only works when the two objects are directly compared. It isn't used inside collections. Is there a way to do this? My specific use case is using MagicMock.assert_has_calls and comparing arguments to the calls.

import unittest


class C:
    def __eq__(self, other):
        return False
        
        
def always_equal(this, that, msg=None):
    return True


class TestCase(unittest.TestCase):

    def test_without_equality_func(self):
        # This passses
        obj = C()
        self.assertEqual(obj, obj)
        
    def test_with_equality_func(self):
        # This fails
        self.addTypeEqualityFunc(C, always_equal)
        obj = C()
        self.assertEqual(obj, obj)      
        
    def test_in_list_with_equality_func(self):
        # This fails
        self.addTypeEqualityFunc(C, always_equal)
        l1 = [C()]
        l2 = [C()]
        self.assertEqual(l1, l2)

Answer 1

addTypeEqualityFunc isn't designed for changing comparison logic. It's designed for providing more detailed error messages when a mismatch is found. The way the API is designed, it wouldn't make sense to use it for subcomparisons.

If you want to patch the logic a class uses to do equality comparisons, then patch the __eq__ method with unittest.mock.patch :

with patch.object(C, '__eq__', always_equal):
    ...