Cumulative sum of non-NA/null values.

        When performing the cumulative summation, any non-NA/null values will
        be skipped. The resulting SparseArray will preserve the locations of
        NaN values, but the fill value will be `np.nan` regardless.

        Parameters
        ----------
        axis : int or None
            Axis over which to perform the cumulative summation. If None,
            perform cumulative summation over flattened array.

        Returns
        -------
        cumsum : SparseArray

def cumsum(self, axis=0, *args, **kwargs):
        """
        Cumulative sum of non-NA/null values.

        When performing the cumulative summation, any non-NA/null values will
        be skipped. The resulting SparseArray will preserve the locations of
        NaN values, but the fill value will be `np.nan` regardless.

        Parameters
        ----------
        axis : int or None
            Axis over which to perform the cumulative summation. If None,
            perform cumulative summation over flattened array.

        Returns
        -------
        cumsum : SparseArray
        """
        nv.validate_cumsum(args, kwargs)

        if axis is not None and axis >= self.ndim:  # Mimic ndarray behaviour.
            raise ValueError("axis(={axis}) out of bounds".format(axis=axis))

        if not self._null_fill_value:
            return SparseArray(self.to_dense()).cumsum()

        return SparseArray(self.sp_values.cumsum(), sparse_index=self.sp_index,
                           fill_value=self.fill_value)