A wrapper for the user-defined handle.

def _updater_wrapper(updater):
    """A wrapper for the user-defined handle."""
    def updater_handle(key, lhs_handle, rhs_handle, _):
        """ ctypes function """
        lhs = _ndarray_cls(NDArrayHandle(lhs_handle))
        rhs = _ndarray_cls(NDArrayHandle(rhs_handle))
        updater(key, lhs, rhs)
    return updater_handle