Should I use HTTP 4xx to indicate HTML form errors?

Question

I just spent 20 minutes debugging some (django) unit tests. I was testing a view POST, and I was expecting a 302 return code, after which I asserted a bunch database entitie

Answer 1

There doesn't appear to be an accepted answer, which to be honest, is a bit surprising. Form validation is such a cornerstone of web development that the fact that there is no response code to illustrate a validation failure seems like a missed opportunity. Particularly given the proliferation of automated testing. It doesn't seem practical to test the response by examining the HTML content for an error message rather than just testing the response code.

I stick by my assertion in the question that 200 is the wrong response code for a request that fails business rules - and that 302 is also inappropriate. (If a form fails validation, then it should not have updated any state on the server, is therefore idempotent, and there is no need to use the PRG pattern to prevent users from resubmitting the form. Let them.)

So, given that there isn't an 'approved' method, I'm currently testing (literally) with my own - 421. I will report back if we run into any issues with using non-standard HTTP status codes.

If there are no updates to this answer, then we're using it in production, it works, and you could do the same.