Debugging redirect cycle error in Django tests

I recently joined a new team with a medium-sized Django project, so naturally I tried to run django-upgrade on it. It modified a lot of URL routes and upgraded the syntax from re_path to path. However, when I ran the tests, I got some failures with the following message:

django.test.client.RedirectCycleError: Redirect loop detected.

I could see the URL being used in the test, but nothing obvious came to mind. The patterns that were changed looked fine, and the problem persisted if I reverted to the pattern in for that URL. I tried to set a breakpoint in the affected view, but it wasn’t reached, where I was expecting it to. It quickly became clear that my test suddenly resolved to a different view, but which one? There are hundreds of routes in the project! I figured that the answer might be in the exception that Django raises in the test.

I wrapped my call to self.client.get(...) in my test into a try/except block and inspected the RedirectCycleError being raised:

class MyViewTest(TestCase):
    def test_page_not_accessible(self):
        url = reverse("my-view")
        try:            response = self.client.get(url, follow=True)
        except RedirectCycleError as exc:            breakpoint()
    assert response.status_code == 404

I ran the test again and inspected the exception from the debugger. the exception has a last_response attribute, which is a HttpResponseRedirectBase, which itself give the URL name that was used by accessing the resolver_match.url_name attribute. Putting it all together:

class MyViewTest(TestCase):
    def test_page_not_accessible(self):
        url = reverse("my-view")
        try:
            response = self.client.get(url, follow=True)
        except RedirectCycleError as exc:
            print(exc.last_response.resolver_match.url_name)

    assert response.status_code == 404

That gave me a URL name which was defined as follows:

from django.urls import re_path

from .views import product_redirect_view

app_name = "site"

urlpatterns = [
    re_path(r"^", product_redirect_view, name="product-redirect"),
]

It’s higher in the list of routes, so higher priority, and is missing a trailing $ hence matching anything. I’m not sure how it worked before, to be honest, but that’s another story I need to figure out next. For now, I’m happy that I found the cause of the problem with debugging.