code: def validate_mandatory_str_fields(self, messages):
          
          FIELDS = ['name', 'download_location', 'verif_code', 'cr_text']
          messages = self.validate_str_fields(FIELDS, False, messages)
  
          return messages
docs: Fields marked as Mandatory and of type string in class
          docstring must be of a type that provides __str__ method.