ContentNotRenderedError when JSON exception swallowed

[rrauenza]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• [ x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [ x] This is not a usage question. (Those should be directed to the discussion group instead.)
• [x ] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• [ x] I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

I've tested this in 3.8.2

Also posted here for future searches: https://stackoverflow.com/questions/53842591/contentnotrenderederror-in-django-rest-framework

I've built a view and serializer where the serializer has a field that is a SerializerMethodField(). In that field's get method I have something like:

def get_result(self, obj):
    result = Result(obj.a, obj.b, obj.c)
    return dict(
        percent_change=result.percent_change,
        [...]
    )

Turns out my result.percent_change sometimes was NaN or Infinity which is not supported by JSON.

The problem is that this root cause was difficult to find because the JSON exception was somehow swallowed and we ended up with a ContentNotRenderedError: The response content must be rendered before it can be accessed. instead.

Regardless whether this gets fixed or not, wanted to post this so people searching for this error realize it's likely a swallowed exception.

Related? #4809

Expected behavior

Instead of ContentNotRenderedError exception, it would be nice to see original exception somehow.

(Or some advice how to find the root cause exception.)

Actual behavior

JSON conversion exception is swallowed.

[lovelydinosaur]

The problem is that this root cause was difficult to find because the JSON exception was somehow swallowed and we ended up with a ContentNotRenderedError: The response content must be rendered before it can be accessed. instead.

I think we'd need to reduce this down to a more specfic example.
Eg. Can this be reproduced just with a plain view that returns a response including a NaN value?

As it stands we can't really form a judgement on if this is something in REST framework that's catching the error, or if it's something within your project that's doing so.

[rrauenza]

I'll see what I can do, but I should also mention I'm using these for my view since it is the view swallowing the error, not the serializer:

           mixins.ListModelMixin,                                   
           mixins.RetrieveModelMixin,                                        
           viewsets.GenericViewSet

I did make a serializer to test this -- just need to make a view:

from rest_framework import serializers

class FooSerializer(serializers.Serializer):

    result = serializers.SerializerMethodField()

    def get_result(self, obj):
        return {
            'nan': float('nan'),
            'num': 1.1111,
        }

[lovelydinosaur]

Going to close this off as stale