Django REST Framework mixin making use of default values when value is None

mixins.py

# coding: utf-8
from __future__ import unicode_literals

from collections import OrderedDict

from rest_framework.fields import SkipField
from rest_framework.relations import PKOnlyObject


class DefaultIfNoneMixin(object):
    """
    DRF doesn't use default value of serializer's field if attribute is present but None.
    Mixin changes that behavior by overriding to_representation method.
    Example usage:
        class MySerializer(DefaultIfNoneMixin, serializers.Serializer):
            a = serializers.CharField(default='default value')
    """

    def to_representation(self, instance):
        """
        Object instance -> Dict of primitive datatypes.
        """
        ret = OrderedDict()
        fields = self._readable_fields

        for field in fields:
            try:
                attribute = field.get_attribute(instance)
            except SkipField:
                continue

            # We skip `to_representation` for `None` values so that fields do
            # not have to explicitly deal with that case.
            #
            # For related fields with `use_pk_only_optimization` we need to
            # resolve the pk value.
            check_for_none = attribute.pk if isinstance(attribute, PKOnlyObject) else attribute
            if check_for_none is None:
                ret[field.field_name] = None
            else:
                ret[field.field_name] = field.to_representation(attribute)

            # The only change. Other parts are directly from DRF.
            if ret[field.field_name] is None:
                try:
                    ret[field.field_name] = field.get_default()
                except SkipField:
                    pass

        return ret