spiral.py

import itertools

steps = itertools.cycle([(0, 1), (1, 0), (0, -1), (-1, 0)])
step = next(steps)

def next_step(x, y):
    global step
    oldv = x, y
    x, y = x + step[0], y + step[1]
    if min(x, y) < 0 or max(x, y) >= N or mat[x][y]:
        step = next(steps)
        return next_step(*oldv)
    return x, y


N = 10
mat = [[0]*N for _ in range(N)]
i, j = 0, -1
for a in range(1, N*N + 1):
    i, j = next_step(i, j)
    mat[i][j] = a

print('\n'.join([' '.join([str(i).rjust(3) for i in row]) for row in mat]))