bad_tests.py

def add(x, y):
    val = 0
    for i in range(x + y):
        val += 1
    return val


import mock
import unittest

from adder import add


class TestAdd(unittest.TestCase):
    def test_add_wrong(self):
        x, y = 1, 3
        with mock.patch.object(add, 'range') as mock_range:
            add.add(x, y)
            mock_range.assert_called_once_with(x + y)