'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]))'

def nonzero(self):

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