mock_patcher_leak.py

import unittest
from unittest import mock

class System:
    def __init__(self):
        self.counter1 = 0
        self.counter2 = 0


    def foo(self):
        self.counter1 += 1

    def bar(self):
        self.counter2 += 1


class SystemTest(unittest.TestCase):

    def setUp(self):
        super().setUp()
        patcher = mock.patch.object(System, 'foo')
        patcher.start()


    def test_bar(self):
        system = System()
        system.bar()
        self.assertEqual(1, system.counter2)

class SystemTest2(unittest.TestCase):
    def setUp(self):
        super().setUp()
        patcher = mock.patch.object(System, 'bar')
        patcher.start()

    def test_foo(self):
        system = System()
        system.foo()
        self.assertEqual(1, system.counter1)


if __name__ == "__main__":
    unittest.main()