Include quality value when parsing Accept header

[Jake Howard]

Requests have an accepts method for identifying the mimetype the user can receive. However, when multiple formats are provided, a ​quality value can be added to show the preferences on format. Handling this value currently needs to be done by hand.

I propose:

1. Documenting the existence of Request.accepted_types
2. Extract the quality value, and store it as MediaType.quality
3. Sort Request.accepted_types by quality value

It would be nice to have an API to answer "I can return these mime types, which would you prefer?", which I guess could be handled manually for now.

[Carlton Gibson]

I think step 3 should read "Sort Request.accepted_types by quality value and specificity", e.g. text/*;q=0.8,text/html;q=0.8 should be sorted text/html first.

This is probably a sensible proposal. I suspect most people are not sending the quality value, so 1 is assumed and it has no effect. Nonetheless why not allow it?

We'd need a new helper API on HttpRequest taking a set of mime types to match against, since the specificity question (text/*) makes it slightly more complex than just returning the first match in accepted_types from the set.

DRF has implementations of this logic that can be used for reference:

• ​rest_framework/negotiation.py#L53-L76
• ​rest_framework/utils/mediatypes.py#L27-L81

[Jake Howard]

I've implemented the core of this feature here: ​PR, sans docs.

I'd appreciate a review, particularly around what API surface to expose.

[Jake Howard]

​PR now contains docs - review away!

[Sarah Boyce <42296566+sarahboyce@…>]

In e161bd46:

Fixed #35631 -- Added HttpRequest.get_preferred_type().