code: def get_subject(self, data):
          
          if not isinstance(data, Mapping):
              return None
          if data.get(self.subject):
              return data.get(self.subject)
          return uuid.uuid4().urn
docs: Try to get a unique ID from the object. By default, this will be
          the 'id' field of any given object, or a field specified by the
          'rdfSubject' property. If no other option is available, a UUID will be
          generated.