proper way python mock __init__() method that returns a fake class

Question

Trying to mock out calls to pyazure library for django testing, but I can't figure out how to mock out the PyAzure class constructor so that it doesn't cause a TypeError. Is there a better way to approach mocking out an access library that generates a connection object?

Anything I've tried other than None generates a TypeError, which means I can't really even begin to test any of the PyAzure connection methods with actual return values. What is the best way to replace a working class with a fake class using mock?

Test Error:

======================================================================
ERROR: test_management_certificate_connect (azure_cloud.tests.ViewsTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/bschott/Source/django-nimbis/apps/azure_cloud/tests.py", line 107, in test_management_certificate_connect
self.cert1.connect()
File "/Users/bschott/Source/django-nimbis/apps/azure_cloud/models.py", line 242, in connect 
    subscription_id=self.subscription.subscription_id)
TypeError: __init__() should return None, not 'FakeAzure'
----------------------------------------------------------------------

tests.py:

 class ViewsTest(TestCase):
    def setUp(self):
    ...
        self.cert1 = ManagementCertificate.objects.create(
            name="cert1",
            subscription=self.subscription1,
            management_cert=File(open(__file__), "cert1.pem"),
            owner=self.user1)
    ...

    class FakeAzure(object):
        """ testing class for azure """
        def list_services(self):
            return ['service1', 'service2', 'service3']
        def list_storages(self):
            return ['storage1', 'storage2', 'storage3']

    @mock.patch.object(pyazure.PyAzure, '__init__')
    def test_management_certificate_connect(self, mock_pyazure_init):
        mock_pyazure_init.return_value = self.FakeAzure()
        self.cert1.connect()
        assert mock_pyazure_init.called

models.py

class ManagementCertificate(models.Model):

    # support connection caching to azure
    _cached_connection = None

    def connect(self):
        """
        Connect to the management interface using these credentials.
        """
        if not self._cached_connection:
            self._cached_connection = pyazure.PyAzure(
                management_cert_path=self.management_cert.path,
                subscription_id=self.subscription.subscription_id)
            logging.debug(self._cached_connection)
        return self._cached_connection

Answer 1

You seem to have a misconception about what __init__() does. Its purpose is to initialise an instance that was already created earlier. The first argument to __init__() is self , which is the instance, so you can see it was already allocated when __init__() is called.

There is a method __new__() that is called before __init__() to create the actual instance. I think it would be much easier, though, to replace the whole class by a mock class, instead of mocking single methods.