Python - 无法修改 Django Rest Framework 中的 serializer.data 字典

Question

I have tried to add a key serializer.data['test'] = 'asdf' , this does not appear to do anything.

I want to transform the representation of a key's value. To do this, I'm trying to use the value to calculate a new value and replace the old one in the dictionary.

This is what I want to accomplish, but I don't know why the value is not being replaced. There are no errors thrown, and the resulting dictionary has no evidence that I've tried to replace anything:

class PlaceDetail(APIView):
    def get(self, request, pk, format=None):
        place = Place.objects.select_related().get(pk=pk)
        serializer = PlaceSerializer(place)
        #serializer.data['tags'] = pivot_tags(serializer.data['tags'])
        serializer.data['test'] = 'asdf'
        print(serializer.data['test'])
        return Response(serializer.data)

Terminal: KeyError: 'test'

I have observed by printing that serializer.data is a dictionary.

I have also tested that the syntax I'm trying to use should work:

>>> test = {'a': 'Alpha'}
>>> test
{'a': 'Alpha'}
>>> test['a']
'Alpha'
>>> test['a'] = 'asdf'
>>> test
{'a': 'asdf'}

How can I properly modify the serializer.data dictionary?

Answer 1

The Serializer.data property returns an OrderedDict which is constructed using serializer._data . The return value is not serializer._data itself.

Thus changing the return value of serializer.data does not change serializer._data member. As a consequence, the following calls to serializer.data are not changed.

# In class Serializer(BaseSerializer)
@property
def data(self):
    ret = super(Serializer, self).data
    return ReturnDict(ret, serializer=self)

# In class ReturnDict(OrderedDict)
def __init__(self, *args, **kwargs):
    self.serializer = kwargs.pop('serializer')
    super(ReturnDict, self).__init__(*args, **kwargs)

You can keep a copy of the return value of serializer.data , which is an ordered dictionary, and manipulate it as you wish.

Example:

# keep the return value of serializer.data
serialized_data = serializer.data
# Manipulate it as you wish
serialized_data['test'] = 'I am cute'
# Return the manipulated dict
return Response(serialized_data)

Why:

If you look at the source code of Django Restframework, you will see that in Serializer class,

• Serializer._data is just a normal dictionary.
• Serializer.data is a method decorated to act like a property. It returns a ReturnDict object, which is a customized class derived from OrderedDict . The returned ReturnDict object is initialized using key/value pairs in Serializer._data .

If Serializer.data returns Serializer._data directly, then your original method will work as you expected. But it won't work since it's returning another dictionary-like object constructed using Serializer._data .

Just keep in mind that the return value of Serializer.data is not Serializer._data , but an ordered dictionary-like object. Manipulating the return value does not change Serializer._data .

I believe the reason why serializer.data does not return serializer._data directly is to avoid accidental change of the data and to return a pretty representation of serializer._data .

Answer 2

You'll want to use SerializerMethodField instead of explicitly overwrite the representations.

Answer 3

Building further on @yuwang's answer, I used SerializerMethodField to modify value of a particular field in the serializer. Here's an example:

The field that I wanted to modify, let us call it is_modifyable . This field is present on the Django Model as models.BooleanField and hence it was not present in the list of fields on serializer definition and simply mentioned in the class Meta: definition under within the serializer definition.

So here's how my code looked before:

# in models.py
# Model definition
class SomeModel(models.Model):
    is_modifyable = models.BooleanField(default=True)

# in serializers.py
# Serializer definition
class SomeModelSerializer(serializers.ModelSerializer):

    class Meta:
        model = SomeModel
        fields = ('is_modifyable',)

As a result of the above, the value for the field is_modifyable was always fetched on the basis of what the value was in the record of SomeModel object. However, for some testing purpose, I wanted the value of this field to be returned as False during the development phase, hence I modified the code to be as follows:

# in models.py
# Model definition (Left unchanged)
class SomeModel(models.Model):
    is_modifyable = models.BooleanField(default=True)

# in serializers.py
# Serializer definition (This was updated)
class SomeModelSerializer(serializers.ModelSerializer):
    # This line was added new
    is_modifyable = serializers.SerializerMethodField(read_only=True)

    class Meta:
        model = SomeModel
        fields = ('is_modifyable',)

    # get_is_modifyable function was added new
    def get_is_modifyable(self, obj) -> bool:
        """
        Dummy method to always return False for test purpose
        Returns: False
        """
        return False

Once the above code was in, the API call always returned the value of serializer field is_modifyable as False.