Escher Escher - 6 months ago 90
Python Question

How to access on ListSerializer parent class in DRF?

I'm getting an error when trying to access
before returning it in the
Response(, status=something)

Getting KeyError when attempting to get a value for field
on serializer

This occurs on all fields (because it turns out I'm trying to access
on the parent and not the child, see below)

The class definition looks like this:

class BulkProductSerializer(serializers.ModelSerializer):

list_serializer_class = CustomProductListSerializer

user = serializers.CharField(source='fk_user.username', read_only=False)

class Meta:
model = Product
fields = (

is a
and has an overridden
method that allows it to correctly handle bulk create and update.

Here's an example view from the bulk Product

def partial_update(self, request):

serializer = self.get_serializer(,
many=isinstance(, list),
if not serializer.is_valid():
return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
return Response(, status=status.HTTP_200_OK)

Trying to access
at the trace (or the line after, obviously) causes the error. Here's the full trace (tl;dr skip below where I diagnose with debugger):

Traceback (most recent call last):
File "/lib/python3.5/site-packages/django/core/handlers/", line 41, in inner
response = get_response(request)
File "/lib/python3.5/site-packages/django/core/handlers/", line 249, in _legacy_get_response
response = self._get_response(request)
File "/lib/python3.5/site-packages/django/core/handlers/", line 187, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/lib/python3.5/site-packages/django/core/handlers/", line 185, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/lib/python3.5/site-packages/django/views/decorators/", line 58, in wrapped_view
return view_func(*args, **kwargs)
File "/lib/python3.5/site-packages/rest_framework/", line 86, in view
return self.dispatch(request, *args, **kwargs)
File "/lib/python3.5/site-packages/rest_framework/", line 489, in dispatch
response = self.handle_exception(exc)
File "/lib/python3.5/site-packages/rest_framework/", line 449, in handle_exception
File "/lib/python3.5/site-packages/rest_framework/", line 486, in dispatch
response = handler(request, *args, **kwargs)
File "/application/siop/views/API/", line 184, in partial_update
return Response(, status=status.HTTP_200_OK)
File "/lib/python3.5/site-packages/rest_framework/", line 739, in data
ret = super(ListSerializer, self).data
File "/lib/python3.5/site-packages/rest_framework/", line 265, in data
self._data = self.to_representation(self.validated_data)
File "/lib/python3.5/site-packages/rest_framework/", line 657, in to_representation
self.child.to_representation(item) for item in iterable
File "/lib/python3.5/site-packages/rest_framework/", line 657, in <listcomp>
self.child.to_representation(item) for item in iterable
File "/lib/python3.5/site-packages/rest_framework/", line 488, in to_representation
attribute = field.get_attribute(instance)
File "/lib/python3.5/site-packages/rest_framework/", line 464, in get_attribute
raise type(exc)(msg)
KeyError: "Got KeyError when attempting to get a value for field `user` on serializer `BulkProductSerializer`.\nThe serializer field might be named incorrectly and not match any attribute or key on the `OrderedDict` instance.\nOriginal exception text was: 'fk_user'."

At the L657 of the traceback (source here) I've got:

iterable = data.all() if isinstance(data, models.Manager) else data
return [
self.child.to_representation(item) for item in iterable

This made me wonder (digging further down in the trace) why the serializer.fields were not available. I suspected it was because the serializer was a
parent, and not a
child, and I was right. In the pdb trace just before returning the

(Pdb) serializer.fields
*** AttributeError: 'CustomProductListSerializer' object has no attribute 'fields'
(Pdb) serializer.child.fields
{'uuid': UUIDField(read_only=False, required=False, validators=[]) ...(etc)}
(Pdb) 'user' in serializer.child.fields
*** KeyError: "Got KeyError when attempting to get a value for field `user` on serializer `BulkProductSerializer`.\nThe serializer field might be named incorrectly and not match any attribute or key on the `OrderedDict` instance.\nOriginal exception text was: 'fk_user'."
{'uuid': '08ec13c0-ab6c-45d4-89ab-400019874c63', ...(etc)}

OK, so what's the right way to get the complete
and return it in the resopnse for the parent serializer class in the situation described by
in my


class CustomProductListSerializer(serializers.ListSerializer):

def save(self):
instances = []
result = []
for obj in self.validated_data:
uuid = obj.get('uuid', None)
if uuid:
instance = get_object_or_404(Product, uuid=uuid)
# Specify which fields to update, otherwise save() tries to SQL SET all fields.
# Gotcha: remove the primary key, because update_fields will throw exception.
# see
update_fields = [k for k,v in obj.items() if k != 'uuid']
for k, v in obj.items():
if k != 'uuid':
setattr(instance, k, v)

if len(instances) > 0:
result += instances

return result

Answer Source

At the point in the trace where I try to access and get the KeyError, I note that only contains key/vaule pairs from the initial_data, not the instance data (hence, I suppose, the KeyError; some model fields' keys are not present as it is a partial_update request). However, does contain all the instance data for the last child in the list.

So, I go to the rest_framework/ source where data is defined:

249    @property
250    def data(self):
251        if hasattr(self, 'initial_data') and not hasattr(self, '_validated_data'):
252            msg = (
253                'When a serializer is passed a `data` keyword argument you '
254                'must call `.is_valid()` before attempting to access the '
255                'serialized `.data` representation.\n'
256                'You should either call `.is_valid()` first, '
257                'or access `.initial_data` instead.'
258            )
259            raise AssertionError(msg)
261        if not hasattr(self, '_data'):
262            if self.instance is not None and not getattr(self, '_errors', None):
263                self._data = self.to_representation(self.instance)
264            elif hasattr(self, '_validated_data') and not getattr(self, '_errors', None):
265                self._data = self.to_representation(self.validated_data)
266            else:
267                self._data = self.get_initial()
268        return self._data

Line 265 is problematic. I can replicate the error by calling serializer.child.to_representation({'uuid': '87956604-fbcb-4244-bda3-9e39075d510a', 'product_code': 'foobar'}) at the breakpoint.

Calling partial_update() works fine on a single instance (because self.instance is set, self.to_representation(self.instance) works). However, for a bulk partial_update() implementation, self.validated_data is missing model fields, and to_representation() won't work, so I won't be able to access the .data property.

One option would be to maintain some sort of self.instances list of Product instances, and override the definition of data on line 265:

self._data = self.to_representation(self.instances)

I'd really prefer an answer from someone more experienced in this sort of problem though, as I'm not sure if that's a sensible solution, hence I'm leaving the bounty open in the hope that someone can suggest something smarter to do.

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