gistfile1.py

class Validated:
    __fields__ = {} # field -> validator list

    def validate(self):
        errors = []
        for field, validators in self.__fields__.iteritems():
            if not hasattr(self, field):
                errors.append("field %s missing" % field)
                continue
            field_errors = []
            value = getattr(self, field)
            for validator in validators:
                try:
                    validator(value)
                except Exception, e:
                    field_errors.append(str(e))
            if field_errors:
                errors.append("%s: %s" % (field, ", ".join(field_errors)))

        if errors:
            assert 0, errors


def is_positive(n):
    assert (type(n) in (int, long, float)) and (n > 0), "not positive"
            
class Tunnel(Validated):
    __fields__ = {
        "status" : [is_positive]
    }

    status = 0
    host = 0
    type = 0

    def save(self):
        self.validate()
        # ...


t = Tunnel()
t.save()