my solution to spiral ascension

spiral_ascension.py

# returns True/False if we need to turn
def at_edge(r, c, board, dir):
    n = len(board) - 1
    dir = dir.upper()

    if (dir == 'N'):
        return (r == 0) or (board[r - 1][c] != 0)
    elif (dir == 'E'):
        return (c == n) or (board[r][c + 1] != 0)
    elif (dir == 'S'):
        return (r == n) or (board[r + 1][c] != 0)
    elif (dir == 'W'):
        return (c == 0) or (board[r][c - 1] != 0)
    else:
        print "ERROR: dir is not N, E, S, W"

# returns the new direction after a turn
def turn(dir, clockwise):
    if (dir == 'N'):
        return 'E' if clockwise else 'W'
    elif (dir == 'E'):
        return 'S' if clockwise else 'N'
    elif (dir == 'S'):
        return 'W' if clockwise else 'E'
    elif (dir == 'W'):
        return 'N' if clockwise else 'S'
    else:
        print "ERROR: dir is not N, E, S, W"

# returns a tuple of the next (r, c) step going straight
def go_straight(r, c, dir):
    if (dir == 'N'):
        return (r - 1, c)
    elif (dir == 'E'):
        return (r, c + 1)
    elif (dir == 'S'):
        return (r + 1, c)
    elif (dir == 'W'):
        return (r, c - 1)
    else:
        print "ERROR: dir is not N, E, S, W"

# prints a spiral starting in the top-left (0, 0)
# bonus second parameter for counter-clockwise spiral
def solution(n, clockwise = True):
    #initialize the board, number list, and row/col pair
    board = [[0 for c in xrange(n)] for r in xrange(n)]
    max_num = n ** 2
    nums = range(1, max_num + 1)
    r = c = 0
    dir = 'E' if clockwise else 'S'

    for i in nums:
        # place the number on board
        board[r][c] = i

        # update r, c for next iteration
        if at_edge(r, c, board, dir): dir = turn(dir, clockwise)
        (r, c) = go_straight(r, c, dir)

    # print a nice-looking board
    width = len(str(max_num))

    for row in board:
        print ' '.join('{num:{fill}{width}}'.format(num=row[i], fill="", width=width) for i in xrange(n))

def main():
    # acceptable values for n
    proper_nums = [str(n) for n in range(36)]
    print ("Hi, welcome to my solution to Spiral Ascenion...")
    while (True):
        n_str = raw_input("Input any number within [0,35] > ").strip()
        if (n_str in proper_nums):
            break
        else:
            print ("Oops! Try again...")
            continue

    clockwise = raw_input("Press Enter now for clockwise or input any other key for counter-clockwise > ")
    clockwise = not bool(clockwise)

    solution(int(n_str), clockwise)

if __name__ == "__main__":
    main()