SQLAlchemy validation hackfest

import ThreadLocalData # location of appropriate TZ info
import sqlalchemy.ext.declarative as declarative
from sqlalchemy import types, Column, Integer, String
from pytz import UTC as _UTC

OrmBase = declarative.declarative_base()

class UTCEnforcedDateTime(types.TypeDecorator):
    """DateTime type to ensure datetime objects are offset-aware UTC."""
    impl = types.DateTime

    def process_bind_param(self, value, engine):
        if (value is not None) and (value.tzinfo != _UTC):
            raise Exception("illegal naive datetime instance!")
        return value

    def process_result_value(self, value, engine):
        if value is not None:
            return value.replace(tzinfo = _UTC)
        return value

class TableDefMixin(object):
    def _EnsureUTC(self, AttrName, dt):
        if dt == None:
            return dt
        assert dt.tzinfo == None, \
               "offset-aware datetime not expected"
        tz = ThreadLocalData.LocalTZ
        #Convert naive time to local time...
        # - normalize is needed to deal with DST funkiness
        dt_tz = tz.normalize(tz.localize(dt))
        #return the time as UTC...
        return dt_tz.astimezone(_UTC)

class User(OrmBase, TableDefMixin):
    __tablename__ = "user"
    id = Column(Integer, primary_key = True, autoincrement = True)
    login = Column(String(100), index = True, unique = True)
    creation_date = Column(UTCEnforcedDateTime, nullable = False)
    last_login = Column(UTCEnforcedDateTime, nullable = True)
    #<snip>

#Nasty hack (?) is here!
utcConversionList = ["creation_date", "last_login"]
User._EnsureUTC.__dict__["__sa_validators__"] = utcConversionList