code: def _get_dynamic_data(self, validated_data):
          
          result = {}
  
          for key in self.initial_data:
              if key not in validated_data:
                  try:
                      field = self.fields[key]
                      # no exception? this is either SkipField or error
                      # in particular, this might be a read-only field
                      # that was mistakingly given a value
                      if not isinstance(field, drf_fields.SkipField):
                          msg = (
                              'Field %s is missing from validated data,'
                              'but is not a SkipField!'
                          ) % key
                          raise AssertionError(msg)
                  except KeyError:  # ok, this is dynamic data
                      result[key] = self.initial_data[key]
          return result
docs: Returns dict of data, not declared in serializer fields.
          Should be called after self.is_valid().