"""
Module implementing message related functions and classes.
"""
from typing import Type, TypeVar

QuestionInst = TypeVar('QuestionInst', bound='Question')


class MessageError(Exception):
    pass


class Question(str):
    """
    A single question with a defined prefix.
    """
    prefix = '=== QUESTION ==='

    def __new__(cls: Type[QuestionInst], string: str) -> QuestionInst:
        """
        Make sure the question string does not contain the prefix.
        """
        if cls.prefix in string:
            raise MessageError(f"Question '{string}' contains the prefix '{cls.prefix}'")
        instance = super().__new__(cls, string)
        return instance