HttpRequest.get_preferred_type misorders types when more specific accepted types have lower q

[Anders Kaseorg]

Consider this example from ​RFC 9110 §12.5.1:

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence that matches the type. For example,

Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
       text/plain;format=fixed;q=0.4, */*;q=0.5

would cause the following values to be associated:

Media Type	Quality Value
text/plain;format=flowed	1
text/plain	0.7
text/html	0.3
image/jpeg	0.5
text/plain;format=fixed	0.4
text/html;level=3	0.7

Django’s HttpRequest.get_preferred_type fails to match this behavior.

>>> from django.conf import settings
>>> from django.http import HttpRequest
>>> settings.configure()
>>> request = HttpRequest()
>>> request.META["HTTP_ACCEPT"] = "text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed, text/plain;format=fixed;q=0.4, */*;q=0.5"
>>> request.get_preferred_type(['text/plain', 'text/plain;format=fixed']) # expected text/plain (0.7 > 0.4)
'text/plain;format=fixed'
>>> request.get_preferred_type(['text/html', 'image/jpeg']) # expected image/jpeg (0.3 < 0.5)
'text/html'
>>> request.get_preferred_type(['text/html', 'text/html;level=3']) # expected text/html;level=3 (0.3 < 0.7)
'text/html'
>>> request.get_preferred_type(['image/jpeg', 'text/html']) # expected image/jpeg (0.5 > 0.3)
'text/html'
>>> request.get_preferred_type(['image/jpeg', 'text/plain;format=fixed']) # expected image/jpeg (0.5 > 0.4)
'text/plain;format=fixed'
>>> request.get_preferred_type(['text/plain;format=fixed', 'text/plain']) # expected text/plain (0.4 < 0.7)
'text/plain;format=fixed'
>>> request.get_preferred_type(['text/plain;format=fixed', 'image/jpeg']) # expected image/jpeg (0.4 < 0.5)
'text/plain;format=fixed'
>>> request.get_preferred_type(['text/plain;format=fixed', 'text/html;level=3']) # expected text/html;level=3 (0.4 < 0.7)
'text/plain;format=fixed'
>>> request.get_preferred_type(['text/html;level=3', 'text/plain;format=fixed']) # expected text/html;level=3 (0.7 > 0.4)
'text/plain;format=fixed'