[Python] Django Rest Framework partial update

I'm trying to implement partial_update with Django Rest Framework but I need some clarification because I'm stuck.

1. 
   Why do we need to specify partial=True?
   In my understanding, we could easily update Demo object inside of partial_update method. What is the purpose of this?
   
   
2. 
   What is inside of serialized variable?
   What is inside of serialized variable in partial_update method? Is that a Demo object? What function is called behind the scenes?
   
   
3. How would one finish the implementation here?

Viewset
​

class DemoViewSet(viewsets.ModelViewSet):
serializer_class = DemoSerializer

def partial_update(self, request, pk=None):
serialized = DemoSerializer(request.user, data=request.data, partial=True)
return Response(status=status.HTTP_202_ACCEPTED)

Serializer
​

class DemoSerializer(serializers.ModelSerializer):
class Meta:
model = Demo
fields = '__all__'

def update(self, instance, validated_data):
print 'this - here'
demo = Demo.objects.get(pk=instance.id)
Demo.objects.filter(pk=instance.id)\
.update(**validated_data)
return demo

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