django rest framework SerializerMethodField bizarre behaviour

Question

When two serializer classes declare a SerializerMethodField with same name, like this:

class APIProfileSerializer(serializers.ModelSerializer):
    def __init__(self, *args, **kwargs):

        super(APIProfileSerializer, self).__init__(*args, **kwargs)
        self.request = request

    def get_image(self, instance):

        if instance.image:
            return self.request.build_absolute_uri(location=instance.image.url)
        return None

    image = serializers.SerializerMethodField()

    # other ProfileSerializer fields

    class Meta:
        model = User
        fields = ('image')



class ProfileSerializer(serializers.ModelSerializer):
    def __init__(self, *args, **kwargs):

        super(ProfileSerializer, self).__init__(*args, **kwargs)
        self.request = request

    def get_image(self, instance):

        if instance.image:
            return self.request.build_absolute_uri(location=instance.image.url)
        return None

    image = serializers.SerializerMethodField()

    # other ProfileSerializer fields

    class Meta:
        model = User
        fields = ('image')

the code

serializer = APIProfileSerializer(instance=request.user, request=request)
return Response(data=serializer.data)

crashes with the following traceback:

Traceback (most recent call last): File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/django/core/handlers/base.py", line 174, in get_response response = self.process_exception_by_middleware(e, request) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/django/core/handlers/base.py", line 172, in get_response response = response.render() File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/django/template/response.py", line 160, in render self.content = self.rendered_content File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/response.py", line 71, in rendered_content ret = renderer.render(self.data, media_type, context) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/renderers.py", line 675, in render context = self.get_context(data, accepted_media_type, renderer_context) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/renderers.py", line 618, in get_context raw_data_put_form = self.get_raw_data_form(data, view, 'PUT', request) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/renderers.py", line 540, in get_raw_data_form content = renderer.render(serializer.data, accepted, context) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/serializers.py", line 503, in data ret = super(Serializer, self).data File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/serializers.py", line 239, in data self._data = self.to_representation(self.instance) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/serializers.py", line 472, in to_representation ret[field.field_name] = field.to_representation(attribute) File "/home/ubuntu/venv/superhq_rest/lib/python3.4/site-packages/rest_framework/fields.py", line 1645, in to_representation return method(value) File "/home/ubuntu/vhosts/superhq_backend/account/serializers.py", line 32, in get_image return self.request.build_absolute_uri(location=instance.image.url) AttributeError: 'NoneType' object has no attribute 'build_absolute_uri'

( my first question is: how to make traceback more readable in markdown? =) )

It's like request hasn't been passed to serializer

however, this

serializer = ProfileSerializer(instance=request.user, request=request)
return Response(data=serializer.data)

and this

serializer = APIProfileSerializer(instance=request.user,request=request)
_data = serializer.data,
return Response(data=_data)

work fine...

Debugging show that BOTH APIProfileSerializer and ProfileSerializer are called, and the crash is caused by calling get_image in ProfileSerializer without passing request to it

So, the question is: WTF?! I didn't call this serializer! I never even mentioned it in my views! How and for what reason is it being called?

I stick to the latter code piece to solve this problem, but there's got to be a better way! Any suggestions?

Answer 1

It appears that DRF's ImageField returns full URL if request has been provided to serializer context, so there is no need for explicit URL construction. Clean and elegant (requires source studying though - official documentation does not mention this)

Answer 2

I kinda solved it like this:

changed

if instance.image:

to

if instance.image and self.request:

so these under-the-hood get_image() calls without a proper self.request do not cause crashes anymore.

Still, it would be great to know why all of the above takes place