访问者模式，一个物体针对不同的访问者，所展现的行为是不同的。
class Visitor(object):
def __init__(self, type_):
self.type_ = type_
class Obj(object):
def __init__(self, visitor):
self.visitor_type = visitor.type_
self.sell_price = 100
def show(self):
if self.visitor_type == 'buyer':
print('to buyer: it sell ￥', self.sell_price)
elif self.visitor_type == 'seller':
print('to seller: it cost ￥', self.sell_price * 0.8)
else:
print("undefined")
def main():
seller = Visitor('seller')
obj = Obj(seller)
obj.show()
buyer = Visitor('buyer')
obj = Obj(buyer)
obj.show()
if __name__ == '__main__':
main()
