nonzero alternative implementation

coo.py

class coo_1d(_data_matrix, ...):  
    ...
    def row(self):
        return np.zeros(self.col.shape[0])
    
    def nonzero(self):
        ...

        nz_mask = A.data != 0
        return (None, A.col[nz_mask])
 
class coo_2d(_data_matrix, ...):  
    ...
    def nonzero(self):
        ...

        A = self.tocoo()
        nz_mask = A.data != 0
        return (A.row[nz_mask], A.col[nz_mask])