Bowling Game module

class GameNotStarted(Exception):
    pass

class GameNotFinished(Exception):
    pass

class GameOver(Exception):
    pass

class BowlingGameSetting(object):
    MAX_ROLLS = 21
    ROLL_PER_FRAME = 2
    STRIKE_POINTS = 10
    STRIKE_BOUNS_ROLLS = 2
    SPARE_POINTS = 10
    SPARE_BONUS_ROLLS = 1

class BowlingGame(object):
    """ Bowling game """

    def __init__(self):
        """ Constructor """
        self.__rolls = []

    def play(self,pinsDown):
        """ update the scores based on the pins down during the roll """

        if len(self.__rolls) >= BowlingGameSetting.MAX_ROLLS:
            raise GameOver
        self.__rolls.append(pinsDown)

    def __canScore(self):
        """ checks to see if we can score """
        if len(self.__rolls) == 0:
            raise GameNotStarted
        if len(self.__rolls) < BowlingGameSetting.MAX_ROLLS - 1:
            raise GameNotFinished

    def __getBonusRollScores(self,startingAt,nItems):
        """ total the scores of bonus rolls """
        itemCount = 0
        bonus = 0
        for item in self.__rolls[startingAt + 1:]:
            if itemCount >= nItems:
                break
            bonus += item
            itemCount += 1
        return bonus

    def __getBonus(self,frameIndex,frame):
        """ calculates the bonus for the provided frame """
        #bonus for spare frame
        if sum(frame) == BowlingGameSetting.SPARE_POINTS:
            return self.__getBonusRollScores((frameIndex * BowlingGameSetting.ROLL_PER_FRAME) + 1,BowlingGameSetting.SPARE_BONUS_ROLLS)
        #bonus for every strike in the frame
        bonus = 0
        for index,roll in enumerate(frame):
            if roll == BowlingGameSetting.STRIKE_POINTS:
                bonus += self.__getBonusRollScores((frameIndex * BowlingGameSetting.ROLL_PER_FRAME) + index,BowlingGameSetting.STRIKE_BOUNS_ROLLS)
        return bonus

    def score(self):
        """ calculates the  score """
        self.__canScore()
        #calculate the score
        score = 0
        for frameIndex,frame in enumerate(zip(*[iter(self.__rolls)]* BowlingGameSetting.ROLL_PER_FRAME)):  #split the rolls into frame.
            score += sum(frame) + self.__getBonus(frameIndex,frame)
        return score