AttributeError: can't set attribute

This answer doesn't address the specifics of this question, but explains the underlying issue.This specific exception "AttributeError: can't set attribute" is raised (see source) when the attribute you're attempting to change is actually a property that doesn't have a setter. If you have access to the library's code, adding a setter would solve the problem.

EDIT: updated source link to new location in the code.

It looks like you don't use self.template in Response class. Try like this:

class Response(HttpResponse):    def __init__(self, template='', calling_context='' status=None):        HttpResponse.__init__(self, get_template(template).render(calling_context), status)

I took a look to django source code I've no idea where template or templates attribute come from in HttpResponse. But I can propose to you to change your test approach and migrate to mock framework. You can rewrite your test like:

@patch("qualified_path_of_response_module.response.Response", spec=Response)def test_should_list_all_users_for_that_specific_sales_office(self,mock_resp):    user_company = CompanyFactory.create()    request = self.mock_request(user_company)    #some other stuff    #calling the view    response = show(request, sales_office_id=sales_office.id)    self.assertTrue(mock_resp.called)    context = mock_resp.call_args[0][2]    self.assertIn(user, context["sales_office_users"])    self.assertNotIn(user2, context["sales_office_users"])

@patch decorator replace your Response() class by a MagicMock() and pass it to your test method as mock_resp variable. You can also use patch as context manager by with construct but decorators are the cleaner way to do it. I don't know if Response is just a stub class for testing but in that case you can patch directly HttpResponce, but it depends from your code.

You can find details about call_args here. Maybe you need to use spec attribute because django make some type checking... but try with and without it (I'm not a django expert). Explore mock framework: it'll give to you lot of powerful tools to make simple tests.