I think, given that WebAPIResponse doesn't necessarily mean you're using resources, that we shouldn't special-case this. Instead, we should just stick it in the headers passed to this.

Something about this sentence is making it hard for me to read. Something about the combination of the comma and the "respectively" maybe?

Should probably be styled as:

allowed_mimetypes = [
    {'list': mime, 'item': mime}
    for mime in ...
]

No need for the list(), and generall the value and the for in a generator are aligned.

Not a huge deal, but rather than loop twice, we should probably just loop once and append to each list.

Blank line before blocks.

Can you go into more detail as to what this code is doing? It's not immediately obvious.

Blank line between these.

Generally, I'd prefer we expand these dictionaries. It's more code, but clearer.

vend_mimetype_pair = {
    'list': None,
    'item': None,
}

Looks like this can still be an and?

Just a few nits that I found - nothing major. Tests pass locally too.

Nit - lines 234, 236, 238, 240 and 242 are misaligned.

Mis-aligned

Do we really want to print the response in this test helper? Or is this leftover debugging stuff?

Ship It!

Couple small things remaining, and then we're done :)

(Dropped the ball on this. We'll get it into the next 1.6 release.)

Should have one key per line:

self.allowed_mimetypes.append({
    'list': None,
    'item': mimetype,
})

Same.

This could be simplified if you set a key based on is_list. Then the supported_mimetypes assignment can be done only once, and use mime[key].

Ship It!