Заполнить матрицу спиралью

def f(n):
    a = [[0 for c in range(n)] for r in range(n)]
    k = 1
    mode = 'right'
    r = 0
    c = 0
    while k <= n**2:
        a[r][c] = k
        if mode == 'right':
            c += 1
            if c == n or a[r][c] != 0:
                c -= 1
                r += 1
                mode = 'down'

        elif mode == 'down':
            r += 1
            if r == n or a[r][c] != 0:
                r -= 1
                c -= 1
                mode = 'left'

        elif mode == 'left':
            c -= 1
            if c < 0 or  a[r][c] != 0:
                c += 1
                r -= 1
                mode = 'up'

        elif mode == 'up':
            r -= 1
            if r < 0 or a[r][c] != 0:
                r += 1
                c += 1
                mode = 'right'

        k += 1

    return a


a = f(5)
for x in a:
    print(x)