def safe_type(self, data, tree):
        """
        Make sure that the incoming data complies with the class type we
        are expecting it to be. In this case, classes that inherit from this
        base class expect data to be of type ``list``.
        """
        if not isinstance(data, list):
            name = self.__class__.__name__
            msg = "did not pass validation against callable: %s" % name
            reason = 'expected a list but got %s' % safe_repr(data)
            raise Invalid(self.schema, tree, reason=reason, pair='value', msg=msg)
Make sure that the incoming data complies with the class type we
        are expecting it to be. In this case, classes that inherit from this
        base class expect data to be of type ``list``.