'Lookup callback to use for imports in the following order:
- custom [create, update]_onxxxx
- default [create, update]_onxxxx
- custom onxxxx
- default onxxxx
NB: Currently only onaccept is actually used'

def get_import_callback(self, tablename, callback):

    callbacks = self.base.get('import_callbacks', [])
    if (tablename in callbacks):
        callbacks = callbacks[tablename]
        if (callback in callbacks):
            return callbacks[callback]
    get_config = current.s3db.get_config
    default = get_config(tablename, callback)
    if default:
        return default
    if (callback[:2] != 'on'):
        callback = callback[7:]
        if (callback in callbacks):
            return callbacks[callback]
        default = get_config(tablename, callback)
        if default:
            return default
