Write a number spiral

main.py

from spiral import write_spiral

write_spiral(7)

# Prints:
#  37  36  35  34  33  32  31
#  38  17  16  15  14  13  30
#  39  18   5   4   3  12  29
#  40  19   6   1   2  11  28
#  41  20   7   8   9  10  27
#  42  21  22  23  24  25  26
#  43  44  45  46  47  48  49

write_spiral(6)

# Prints:
#  26  25  24  23  22  21
#  27  10   9   8   7  20
#  28  11   2   1   6  19
#  29  12   3   4   5  18
#  30  13  14  15  16  17
#  31  32  33  34  35  36

spiral.py

def write_row(n, row):
    def tl_corner(n): return (n-1)**2 + 1
    def tr_corner(n): return tl_corner(n) - (n-1)
    def br_corner(n): return n**2
    def bl_corner(n): return br_corner(n) - (n-1)
    
    if row == 0:
        for i in range(tl_corner(n), tr_corner(n)-1, -1):
            write(i)
    elif row == n-1:
        for i in range(bl_corner(n), br_corner(n)+1):
            write(i)
    else:
        write(tl_corner(n) + row)
        write_row(n-2, row-1)
        write(tr_corner(n) - row)

def write_spiral(n):
    for i in range(n):
        write_row(n, i)
        print

def write(n):
    print " %2d" % n,