fantasy_football.py

    class FantasyTeam(object):
       
        def __init__(self):
            self._roster = {'RB' : None,
                            'QB' : None,
                            ... ,
                            'WR': None}
            self._points = 0.0
       
        def draft(self, player):
            if player.position not in self._roster:
                 # update player name
            raise RosterPositionFilledError(self._roster[player.position], player)

    ....


    class RosterPositionFilledError(Exception):
        ERROR_MSG_FRMT = "Roster position of {} already filled by {}. Cannot add {}"

        def __init__(self, roster_player, attempted_player):
             error_msg = ERROR_MSG_FRMT.format(roster_player.position, roster_player, attempted_player)
             super(RosterPositionFilledError, self).__init__(error_msg)