def METHOD_NAME(self, mock_blob_service_factory):
    mock_blob_service_client_instance = Mock()
    mock_blob_service_factory.from_connection_string.return_value = mock_blob_service_client_instance
    backend = AzureBlockBlobBackend(app=self.app, url=self.url)
    # ensure container gets created on client access...
    assert mock_blob_service_client_instance.create_container.call_count == 0
    assert backend._blob_service_client is not None
    assert mock_blob_service_client_instance.create_container.call_count == 1
    # ...but only once per backend instance
    assert backend._blob_service_client is not None
    assert mock_blob_service_client_instance.create_container.call_count == 1