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

docs/ref/request-response.txt

@@ -425,10 +425,48 @@ Methods
     Returns ``True`` if the request is secure; that is, if it was made with
     HTTPS.
 
+.. method:: HttpRequest.get_preferred_type(media_types)
+
+    .. versionadded:: 5.2
+
+    Returns the preferred mime type from ``media_types``, based on the
+    ``Accept`` header, or ``None`` if the client does not accept any of the
+    provided types.
+
+    Assuming the client sends an ``Accept`` header of
+    ``text/html,application/json;q=0.8``:
+
+    .. code-block:: pycon
+
+        >>> request.get_preferred_type(["text/html", "application/json"])
+        "text/html"
+        >>> request.get_preferred_type(["application/json", "text/plain"])
+        "application/json"
+        >>> request.get_preferred_type(["application/xml", "text/plain"])
+        None
+
+    Most browsers send ``Accept: */*`` by default, meaning they don't have a
+    preference, in which case the first item in ``media_types`` would be
+    returned.
+
+    Setting an explicit ``Accept`` header in API requests can be useful for
+    returning a different content type for those consumers only. See
+    :ref:`content-negotiation-example` for an example of returning
+    different content based on the ``Accept`` header.
+
+    .. note::
+
+        If a response varies depending on the content of the ``Accept`` header
+        and you are using some form of caching like Django's
+        :mod:`cache middleware <django.middleware.cache>`, you should decorate
+        the view with :func:`vary_on_headers('Accept')
+        <django.views.decorators.vary.vary_on_headers>` so that the responses
+        are properly cached.
+
 .. method:: HttpRequest.accepts(mime_type)
 
-    Returns ``True`` if the request ``Accept`` header matches the ``mime_type``
-    argument:
+    Returns ``True`` if the request's ``Accept`` header matches the
+    ``mime_type`` argument:
 
     .. code-block:: pycon
 
@@ -436,17 +474,10 @@ Methods
         True
 
     Most browsers send ``Accept: */*`` by default, so this would return
-    ``True`` for all content types. Setting an explicit ``Accept`` header in
-    API requests can be useful for returning a different content type for those
-    consumers only. See :ref:`content-negotiation-example` of using
-    ``accepts()`` to return different content to API consumers.
+    ``True`` for all content types.
 
-    If a response varies depending on the content of the ``Accept`` header and
-    you are using some form of caching like Django's :mod:`cache middleware
-    <django.middleware.cache>`, you should decorate the view with
-    :func:`vary_on_headers('Accept')
-    <django.views.decorators.vary.vary_on_headers>` so that the responses are
-    properly cached.
+    See :ref:`content-negotiation-example` for an example of using
+    ``accepts()`` to return different content based on the ``Accept`` header.
 
 .. method:: HttpRequest.read(size=None)
 .. method:: HttpRequest.readline()

docs/releases/5.2.txt

@@ -226,7 +226,8 @@ Models
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 
-* ...
+* The new :meth:`.HttpRequest.get_preferred_type` method can be used to query
+  the preferred media type the client accepts.
 
 Security
 ~~~~~~~~
@@ -309,6 +310,9 @@ Miscellaneous
 
 * The minimum supported version of ``gettext`` is increased from 0.15 to 0.19.
 
+* ``HttpRequest.accepted_types`` is now sorted by the client's preference, based
+  on the request's ``Accept`` header.
+
 .. _deprecated-features-5.2:
 
 Features deprecated in 5.2

docs/topics/class-based-views/generic-editing.txt

@@ -273,3 +273,56 @@ works with an API-based workflow as well as 'normal' form POSTs::
     class AuthorCreateView(JsonableResponseMixin, CreateView):
         model = Author
         fields = ["name"]
+
+The above example assumes that if the client supports ``text/html``, that they
+would prefer it. However, this may not always be true. When requesting a
+``.css`` file, many browsers will send the header
+``Accept: text/css,*/*;q=0.1``, indicating that they would prefer CSS, but
+anything else is fine. This means ``request.accepts("text/html") will be
+``True``.
+
+To determine the correct format, taking into consideration the client's
+preference, use :func:`django.http.HttpRequest.get_preferred_type`::
+
+    class JsonableResponseMixin:
+        """
+        Mixin to add JSON support to a form.
+        Must be used with an object-based FormView (e.g. CreateView).
+        """
+
+        accepted_media_types = ["text/html", "application/json"]
+
+        def dispatch(self, request, *args, **kwargs):
+            if request.get_preferred_type(self.accepted_media_types) is None:
+                # No format in common.
+                return HttpResponse(
+                    status_code=406, headers={"Accept": ",".join(self.accepted_media_types)}
+                )
+
+            return super().dispatch(request, *args, **kwargs)
+
+        def form_invalid(self, form):
+            response = super().form_invalid(form)
+            accepted_type = request.get_preferred_type(self.accepted_media_types)
+            if accepted_type == "text/html":
+                return response
+            elif accepted_type == "application/json":
+                return JsonResponse(form.errors, status=400)
+
+        def form_valid(self, form):
+            # We make sure to call the parent's form_valid() method because
+            # it might do some processing (in the case of CreateView, it will
+            # call form.save() for example).
+            response = super().form_valid(form)
+            accepted_type = request.get_preferred_type(self.accepted_media_types)
+            if accepted_type == "text/html":
+                return response
+            elif accepted_type == "application/json":
+                data = {
+                    "pk": self.object.pk,
+                }
+                return JsonResponse(data)
+
+.. versionchanged:: 5.2
+
+    The :meth:`.HttpRequest.get_preferred_type` method was added.