code: def serialize(self,
                    node: SchemaNode,
                    appstruct: Union[PotentialDatetimeType,
                                     ColanderNullType]) \
              -> Union[str, ColanderNullType]:
          
          if not appstruct:
              return colander.null
          try:
              appstruct = coerce_to_pendulum(appstruct,
                                             assume_local=self.use_local_tz)
          except (ValueError, ParserError) as e:
              raise Invalid(
                  node,
                  "{!r} is not a pendulum.DateTime object; error was "
                  "{!r}".format(appstruct, e))
          return appstruct.isoformat()
docs: Serializes Python object to string representation.