Short Python script that checks whether a matrix fulfills the triangle inequality

check-triangle-inequality.py

def check_triangle_inequality(D):
    """ Returns true iff the matrix D fulfills
    the triangle inequaltiy.
    """
    n = len(D)
    valid = True
    for i in range(n):
        for j in range(i, n):
            for k in range(n):
                if k == j or k == i:
                    continue
                if D[i][j] > D[i][k] + D[k][j]:
                    print('Invalid triple:', i, k, j)
                    valid = False
    return valid


if __name__ == '__main__':
    D = [[0, 3, 5, 9, 2],
         [3, 0, 4, 7, 1],
         [5, 4, 0, 8, 6],
         [9, 7, 8, 0, 9],
         [2, 1, 6, 9, 0]]
    print(check_triangle_inequality(D))