'return a MapperProperty associated with the given key.'

def get_property(self, key, _configure_mappers=True):

    if (_configure_mappers and Mapper._new_mappers):
        configure_mappers()
    try:
        return self._props[key]
    except KeyError:
        raise sa_exc.InvalidRequestError(("Mapper    '%s'    has    no    property    '%s'" % (self, key)))
