class ExposedException(Exception):
    pass

class CompoundException(ExposedException):

    def __init__(self, exceptions=None):

        self.exceptions = []

        if exceptions is not None:
            self.extend(exceptions)

    @property
    def message(self):
        msg = "There were %d errors:" % len(self)

        for exception in self:
            msg += f"\n{str(exception)}"

        return msg

    def append(self, exception):

        if not issubclass(exception.__class__, ExposedException):
            raise TypeError("CompoundException members MUST be derived from ExposedException")

        self.exceptions.append(exception)

    def extend(self, exceptions):

        for exception in exceptions:

            if not issubclass(exception.__class__, ExposedException):
                raise TypeError("CompoundException members MUST be derived from ExposedException")

        self.exceptions.extend(exceptions)

    def __len__(self):
        return self.exceptions.__len__()

    def __iter__(self):
        return self.exceptions.__iter__()