def test_list_buckets(self): """This is replacing a private member with a mock""" # setup foobar = Foobar() s3_mock = MagicMock() s3_mock.list_buckets.return_value = {'bucketa', 'bucketb'} foobar.s3 = s3_mock # exercise result = foobar._list_buckets() # verify self.assertIn('bucketa', result)