3-2.py

santa_x, santa_y = 0, 0
robot_x, robot_y = 0, 0
visited_santa = [(santa_x, santa_y)]
visited_robot = [(robot_x, robot_y)]

with open('day3.txt', 'r') as f:
    s = f.readline()
    santas_turn = True
    for direction in s:
        if santas_turn:
            if direction == '>':
                santa_x += 1
            elif direction == '<':
                santa_x -= 1
            elif direction == '^':
                santa_y += 1
            elif direction == 'v':
                santa_y -= 1
            visited_santa.append((santa_x, santa_y))
        else:
            if direction == '>':
                robot_x += 1
            elif direction == '<':
                robot_x -= 1
            elif direction == '^':
                robot_y += 1
            elif direction == 'v':
                robot_y -= 1
            visited_robot.append((robot_x, robot_y))
        santas_turn = not santas_turn

print len(set(visited_santa + visited_robot))