How to Assert A Method in Mocked Object Has Been Called?

I a learning python mock, and having problem to check if a method on a mocked object has been called.

The code I want to test is this:

def main(flags, directory_service):
  if flags.has_schema:
    directory_service.HasSchema(SCHEMA_NAME)
    sys.exit(1)

My test code is:

def testHaltsAfterCheckingSchema(self):
  mock_flags = self._CreateDefaultFlags()
  mock_flags.has_schema = True
  mock_directory_service = MagicMock()

  with self.assertRaises(SystemExit) as cm:
    key_distributor.main(mock_flags, mock_directory_service)
  self.assertTrue(mock_directory_service.HasSchema.assert_called_with(key_distributor.SCHEMA_NAME))
  self.assertEqual(cm.exception.code, 1)

And when I run it, I get this exception:

FAIL: testHaltsAfterCheckingSchema (test123.Test123)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test123.py", line 34, in testHaltsAfterCheckingSchema
    self.assertTrue(mock_directory_service.HasSchema.assert_called_with(key_distributor.SCHEMA_NAME))
AssertionError: None is not true