Django Rest Framework: what happened to my default Renderer? - json

Question

I would like calls to /contacts/1.json to return json, 1.api to return browsableAPI, and calls with format=None aka /contacts/1/ to return a template where we call render_form. This way end-users can have pretty forms, and developers can use the .api format, and ajax/apps etc use .json. Seems like a common use case but something isn't clicking for me here in DRF...
Struggling with how DRF determines the Renderer used when no format is given. I found and then lost some info here on stack exchange that basically said to split the responses based on format. Adding the TemplateHTMLRenderer caused all sorts of pain. I had tried to split based on format but that is giving me JSON error below.
I don't understand the de facto way to define what renderer should be used. Especially when no format is provided. I mean, it "just works" when using Response(data). And I can get the TemplateHTMLRenderer to work but at the cost of having no default Renderer.
GET /contacts/1/ Gives the error:
<Contact: Contact object> is not JSON serializable
Using this code:
class ContactDetail(APIView):
permission_classes = (permissions.IsAuthenticatedOrReadOnly,
IsOwnerOrReadOnly,)
queryset = Contact.objects.all()
renderer_classes = (BrowsableAPIRenderer, JSONRenderer, TemplateHTMLRenderer,)
"""
Retrieve, update or delete a contact instance.
"""
def get_object(self, pk):
try:
return Contact.objects.get(pk=pk)
except Contact.DoesNotExist:
raise Http404
def get(self, request, pk, format=None):
contact = self.get_object(pk)
serializer = ContactSerializer(contact)
if format == 'json' or format == "api":
return Response(serializer.data)
else:
return Response({'contact': contact, 'serializer':serializer}, template_name="contact/contact_detail.html")
But GET /contacts/1.json , 1.api, or 1.html ALL give me the correct output. So it seems that I have created an issue with the content negotiation for the default i.e. format=None
I must be missing something fundamental. I have gone through the 2 tutorials and read the Renderers docs but I am unclear on what I messed up here as far as the default. I am NOT using the DEFAULT_RENDERERS in settings.py, didn't seem to make a difference if in default or inside the actual class as shown above.
Also if anyone knows a way to use TemplateHTMLRenderer without needing to switch on format value, I'm all ears.
EDIT: IF I use
if format == 'json' or format == "api" or format == None:
return Response(serializer.data)
else:
return Response({'contact': contact, 'serializer':serializer},
Then I am shown the browsable API by default. Unfortunately, what I want is the Template HTML view by default, which is set to show forms for end users. I would like to keep the .api format for developers.

Answer 1

TL; DR: Check the order of your renderers - they are tried in order of declaration until a content negotiation match or an error occurs.
Changing the line
renderer_classes = (BrowsableAPIRenderer, JSONRenderer, TemplateHTMLRenderer, )
to
renderer_classes = (TemplateHTMLRenderer, BrowsableAPIRenderer, JSONRenderer, )
Worked for me. I believe the reason is because the content negotiator starts at the first element in the renderer classes tuple when trying to find a renderer. When I have format==None, I'm thinking there is nothing else for DRF to go on, so it assumes I mean the "default" which seems to be the first in the tuple.
EDIT: So, as pointed out by #Ross in his answer, there is also a global setting in the settings.py for the project. If I remove my class level renderer_classes declaration and instead use this in settings.py
# ERROR
REST_FRAMEWORK = {
'DEFAULT_RENDERER_CLASSES': (
'rest_framework.renderers.BrowsableAPIRenderer',
'rest_framework.renderers.JSONRenderer',
'rest_framework.renderers.TemplateHTMLRenderer',
)
}
Then I get a (different) JSON error. However, as long as
'rest_framework.renderers.BrowsableAPIRenderer',
is not listed first, for example:
# SUCCESS, even though JSON renderer is checked first
'DEFAULT_RENDERER_CLASSES': (
'rest_framework.renderers.JSONRenderer',
'rest_framework.renderers.TemplateHTMLRenderer',
'rest_framework.renderers.BrowsableAPIRenderer',
)
So if we hit BrowsableAPIRenderer before we try TemplateHTMLRenderer then we get an error - whether or not we are relying on renderer_classes or DEFAULT_RENDERER_CLASSES. I imagine it passes through JSONRenderer gracefully but for whatever reason BrowsableAPIRenderer raises an exception.
So I have simplified my view code after analyzing this...
def get(self, request, pk, format=None):
contact = self.get_object(pk)
serializer = ContactSerializer(contact)
if format == None:
return Response({'contact': contact, 'serializer':serializer}, template_name="contact/contact_detail.html")
else:
return Response(serializer.data)
..which better reflects what I was originally trying to do anyway.

Answer 2

When I look at the source code, the priority seems to be the order of the renderers specified in the DEFAULT_RENDERER_CLASSES parameter in settings.py:
REST_FRAMEWORK = {
'DEFAULT_RENDERER_CLASSES': (
'rest_framework.renderers.JSONRenderer',
'rest_framework.renderers.TemplateHTMLRenderer',
),
'DEFAULT_PARSER_CLASSES': (
'rest_framework.parsers.JSONParser',
'rest_framework.parsers.TemplateHTMLRenderer',
)
}
So, if you specify a bunch of renderer classes, the first renderer that is valid will be selected based on if it is valid for the request given the .json/.api/.html extension and the Accept: header (not content-type, as I said in the comment on your question).