          """
          ret = self._dict_class()
          ret.fields = {}
  
          fields = self.get_fields(nested=bool(self.opts.depth))
          for field_name, field in fields.items():
              key = self.get_field_key(field_name)
              value = field.field_to_native(obj, field_name)
              ret[key] = value
              ret.fields[key] = field
 
</s> remove         fields = self.get_fields(nested=bool(self.opts.depth))
 </s> add         fields = self.get_fields() </s> remove         fields = self.get_fields(nested=bool(self.opts.depth))
 </s> add         fields = self.get_fields() </s> remove         fields = self.default_fields(nested)
        for key, val in fields.items():
 </s> add         for key, val in self.default_fields.items():
        fields = self.get_fields()
          """
          ret = self._dict_class()
          ret.fields = {}
  
        fields = self.get_fields()
          for field_name, field in fields.items():
              key = self.get_field_key(field_name)
              value = field.field_to_native(obj, field_name)
              ret[key] = value
              ret.fields[key] = field
keep keep keep keep replace keep keep keep keep keep
 Cache default fields per serializer instance for improved performance 
rest_framework/serializers.py