Asserting that __init__ was called with right arguments

I'm using python mocks to assert that a particular object was created with the right arguments. This is how my code looks:

class Installer:
    def __init__(foo, bar, version):
        # Init stuff
        pass
    def __enter__(self):
        return self

    def __exit__(self, type, value, tb):
        # cleanup
        pass

    def install(self):
        # Install stuff
        pass

class Deployer:
    def deploy(self):
        with Installer('foo', 'bar', 1) as installer:
            installer.install()

Now, I want to assert that installer was created with the right arguments. This is the code I have so far:

class DeployerTest(unittest.TestCase):
    @patch('Installer', autospec=True)
    def testInstaller(self, mock_installer):
        deployer = Deployer()
        deployer.deploy()

        # Can't do this :-(
        mock_installer.__init__.assert_called_once_with('foo', 'bar', 1)

This is the error I get:

  File "test_deployment.py", line .., in testInstaller
    mock_installer.__init__.assert_called_once_with('foo', 'bar', 1)
AttributeError: 'function' object has no attribute 'assert_called_once_with'