how to unittest the template variables passed to jinja2 template from webapp2 request handler

I'm trying to test my webapp2 handlers. To do this, I thought it would be a good idea to send a request to the handler e.g.:

request = webapp2.Request.blank('/')
# Get a response for that request.
response = request.get_response(main.app)

The problem is, response is mostly just a bunch of HTML etc.

I want to look at what was passed to my jinja2 template from the handler before it was turned into HTML.

I want my test to get at the state within the handler class code. I wan't to be able to see what certain variables looked like in the response handler, and then I want to see what the dict templates looks like before it was passed to render_to_response()

I want to test these variables have the correct values.

Here is my test code so far, but I'm stuck because response = request.get_response() just gives me a bunch of html and not the raw variables.

import unittest
import main
import webapp2

class DemoTestCase(unittest.TestCase):
    def setUp(self):
        pass

    def tearDown(self):
        pass

    def testNothing(self):
        self.assertEqual(42, 21 + 21)

    def testHomeHandler(self):
        # Build a request object passing the URI path to be tested.
        # You can also pass headers, query arguments etc.
        request = webapp2.Request.blank('/')
        # Get a response for that request.
        response = request.get_response(main.app)

        # Let's check if the response is correct.
        self.assertEqual(response.status_int, 200)
        self.assertEqual(response.body, 'Hello, world!')


if __name__ == '__main__':
    unittest.main()

and here is my handler:

class HomeHandler(BaseHandler):
    def get(self, file_name_filter=None, category_filter=None):
        file_names = os.listdir('blog_posts')
        blogs = []

        get_line = lambda file_: file_.readline().strip().replace("<!--","").replace("-->","")

        for fn in file_names:
            with open('blog_posts/%s' % fn) as file_:
                heading = get_line(file_)
                link_name = get_line(file_)
                category = get_line(file_)

            date_ = datetime.strptime(fn.split("_")[0], "%Y%m%d")

            blog_dict = {'date': date_, 'heading': heading,
                         'link_name': link_name,
                         'category': category,
                         'filename': fn.replace(".html", ""),
                         'raw_file_name': fn}

            blogs.append(blog_dict)

        categories = Counter(d['category'] for d in blogs)
        templates = {'categories': categories,
                     'blogs': blogs,
                     'file_name_filter': file_name_filter,
                     'category_filter': category_filter}

        assert(len(file_names) == len(set(d['link_name'] for d in blogs)))

        self.render_template('home.html', **templates)

and here is my basehandler:

class BaseHandler(webapp2.RequestHandler):
    @webapp2.cached_property
    def jinja2(self):
        return jinja2.get_jinja2(app=self.app)

    def render_template(self, filename, **kwargs):
        #kwargs.update({})
        #TODO() datastore caching here for caching of (handlername, handler parameters, changeable parameters, app_upload_date)
        #TODO() write rendered page to its own html file, and just serve that whole file. (includes all posts). JQuery can show/hide posts.
        self.response.write(self.jinja2.render_template(filename, **kwargs))

Perhaps I have got the wrong idea of how to do unit testing, or perhaps I should have written my code in a way that makes it easier to test? or is there some way of getting the state of my code?

Also if someone were to re-write the code and change the variable names, then the tests would break.

You can mock BaseHandler.render_template method and test its parameters.

See this question for a list of popular Python mocking frameworks.