Question

I’m working on a REST API and I really like the idea of using content negotiation to determine what representations to send. My application is based on the Flask framework and so naturally I am working with the mimerender package. I have resource variant selection working for HTML, JSON, and XML. But then I tried it with a bogus mimetype, like application/foobar. I expected to see a 406 error code, but instead I got a 200 response code and the XML representation.

Looking at the source code, it appears that mimerender defaults to whatever mimetype is first in its list of mimetypes, which is XML at the moment.

My question is 2 parts:

1. The guy who wrote mimerender (I hope he reads this question) knows what he is doing, and he must have deliberately chosen to provide a default representation rather than a 406 error code for some good reason. What is the reason behind sending some (kinda random) representation rather than telling a client that you just don’t have what they’re asking for?

2. Assuming that I stubbornly don’t want to send a default representation and that I prefer to send a 406 error instead, how can I do this within the confines of Flask and mimerender? One possibility I can think of is to register a fake mimetype, set that as the default, and call abort(406) in its handler. But that seems hacky.

Answer 1

I think I had not given this case enough thought. According to the spec, it’s still ok to return an unacceptable response:

HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the
request. In some cases, this may even be preferable to sending a
406 response. User agents are encouraged to inspect the headers of
an incoming response to determine if it is acceptable.

I have just added an optional boolean flag which makes mimerender fail with 406 instead. Hopefully that will cover your use case as well.