No 'Access-Control-Allow-Origin' header error triggered accidentally when using format_suffix_patterns in Django REST framework

After I set up my Django backend with django-cors-headers, I tried to access the api endpoint at http://127.0.0.1:8000/api from the front-end (which is served from http://localhost:8080) by a GET request.

However, in the console of the browser, the typical CORS error was thrown like this:

Access to XMLHttpRequest at 'http://127.0.0.1:8000/api' from origin 'http://localhost:8080' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

However, I’m pretty sure that my setup of CORS on the backend side is correct, and I did not find any useful solution to my problem.

Finally, I figured out that, since I used format_suffix_patterns for my urlpatterns, a call to http://127.0.0.1:8000/api will result in GETting the rendered html page.

Therefore, by changing the query to GET http://127.0.0.1:8000/api.json the haunting CORS problem is gone.