如何改进这个面向对象的Python代码？

Question

我是一个新的Python，并没有很好的知识OOD。我完成了以下任务，但我觉得代码不好，我不知道如何改进它。有人能帮我吗？你知道吗

You need to create the foundations of an e-commerce engine for a B2C (business-to-consumer) retailer. You need to have a class for a customer called User, a class for items in inventory called Item, and a shopping cart class called Cart. Items go in Carts, and Users have one Carts. but, multiple items can go into Carts, including more than one of any single item.

代码如下所示：

class User(object):
    def __init__(self):
        self.__cart = Cart()

    def BuyGoods(self, gname, gnum, item):
        self.__cart.AddItem(gname, gnum, item)

    def ShowMygoods(self):
        return self.__cart.ShowCart()


class Cart(object):
    def __init__(self):
        self.__Goodslist = {}

    def AddItem(self,goods,goodsnum, item):
        if item.SaleGoods(goods,goodsnum) == 0:        #class interaction
            try:
                self.__Goodslist[goods] += goodsnum
            except:
                self.__Goodslist.update({goods:goodsnum})

    def DelItem(self,goods,goodsnum, item):
        try:
            self.__Goodslist[goods] -= goodsnum

            if self.__Goodslist[goods] < 0:
                self.__Goodslist[goods] += goodsnum
                print "Warnning:there is only %d %s in Cart" % (self.__Goodslist[goods],goods)
                return -1

            elif self.__Goodslist[goods] == 0:
                self.__Goodslist.pop(goods)

            item.AddGoods(goods,goodsnum)   #class interaction
            return 0

        except:
            print "Error:there are no %s in Cart" % (goods)
            return -1

    def ShowCart(self):
        return self.__Goodslist 


class Item(object):
    def __init__(self):
        self.__itemdict = {}

    #@classmethod
    def AddGoods(self,goodsname,goodsnum):
        try:
            self.__itemdict[goodsname] += goodsnum
        except:
            self.__itemdict.update({goodsname:goodsnum})

    #@classmethod
    def SaleGoods(self,goodsname,goodsnum):
        try:
            self.__itemdict[goodsname] -= goodsnum

            if self.__itemdict[goodsname] < 0:
                self.__itemdict[goodsname] += goodsnum
                print "Warnning:there is only %d %s in Item" % (self.__itemdict[goodsname],goodsname)
                return -1

            elif self.__itemdict[goodsname] == 0:
                self.__itemdict.pop(goodsname)
            return 0

        except:
            print "Error:there are no %s in Item" % (goodsname)
            return -1


    def ShowItem(self):
        return self.__itemdict



if __name__ == '__main__':
    item1 = Item()
    item1.AddGoods('apple', 10)
    item1.AddGoods('pear', 5)
    print "item1 have %s" % item1.ShowItem()


    item2 = Item()
    item2.AddGoods('apple', 10)
    print "item2 have %s" % item2.ShowItem()


    User1 = User()
    User1.BuyGoods('apple', 7, item1)
    print "User1 have: %s" % User1.ShowMygoods()

    print "item1 have %s" % item1.ShowItem()