spmatrix nonzero implementation

base.py

    def nonzero(self):
        """nonzero indices

        Returns a tuple of arrays (row,col) containing the indices
        of the non-zero elements of the matrix.

        Examples
        --------
        >>> from scipy.sparse import csr_matrix
        >>> A = csr_matrix([[1,2,0],[0,0,3],[4,0,5]])
        >>> A.nonzero()
        (array([0, 0, 1, 2, 2]), array([0, 1, 2, 0, 2]))

        """

        # convert to COOrdinate format
        A = self.tocoo()
        nz_mask = A.data != 0
        return (A.row[nz_mask], A.col[nz_mask])