Module ma

ma.py

class Parent(object):
    pass

class MyError(Exception):
    pass

class isSecondField(object):
    def __init__(self, value):
        self.value = value

    def __set__(self, obj, value):
        raise AttributeError

    def __get__(self, obj, type=None):
        return self.value


class First(Parent):
    isSecond = isSecondField(False)

    @staticmethod
    def isFirst():
        return True


class Second(Parent):
    isSecond = isSecondField(True)

    @staticmethod
    def isFirst():
        return False

class A(First):

    def __init__(self):
        self.i = 3

    @staticmethod
    def fnc(a):
        if a == 7:
            raise MyError("Error text")
        return a * a * 3