docs: def to_python(self, value):
          """Convert value if needed."""
          if isinstance(value, GroupDescriptor):
              value = value._value  # pylint: disable=protected-access
  
          result = {}
          for name, field in self.fields.items():
              result[name] = field.to_python(value.get(name, None))
  
          return GroupDescriptor(result)