Get a deep copy NDArray of the i-th aux data array associated with the
        BaseSparseNDArray.

        This function blocks. Do not use it in performance critical code.

def _aux_data(self, i):
        """ Get a deep copy NDArray of the i-th aux data array associated with the
        BaseSparseNDArray.

        This function blocks. Do not use it in performance critical code.
        """
        self.wait_to_read()
        hdl = NDArrayHandle()
        check_call(_LIB.MXNDArrayGetAuxNDArray(self.handle, i, ctypes.byref(hdl)))
        return NDArray(hdl)