text: class ValidationError(Exception):
      """
      A L{ValidationError} is raised by L{Field.extractValue}.
      """
  
      def __init__(self, message: str) -> None:
          """
          Initialize a L{ValidationError} with a message to show to the user.
          """
          super().__init__(message)
          self.message = message
  
  
  class ValueAbsent(ValidationError):
      """
      A value was required but none was supplied.
      """
  