为什么我的@property装饰器不起作用？

Question

I'm working on this little Class, playing with the @property decorators. For some reason it's not working right.

class Card:
    def __init__(self, rank, suit):
        self.rank = rank
        self.suit = suit

    @property 
    def rank(self):
        return self._rank

    @rank.setter
    def rank(self, value):
        valid_ranks = ["2", "3", "4", "5", "6", "7", "8", "9", "J", "Q", "K", "A"]
        if valid_ranks.index(value):
            self._rank = value

    @property
    def suit(self):
        return self._suit

    @suit.setter
    def suit(self,value):
        self._suit = value

    def show(self):
        print "{}{}".format(self.rank, self.suit)

I initialize an object like so: Card("cheese", "ball") , and presumably this should throw an error. Python happily just rolls with it though and assigns "cheese" to rank. What's going on here? All other calls to the rank setter through assignment syntax seem to happily ignore the setter I've put in place. I'm running Python 2.7.5.

Answer 1

You're using old-style classes. As noted in the documentation , properties don't work with old-style classes. Derive your classes from object . Eg:

class Card(object):
    ...

In Python 3 you don't need to do this, because Python 3 only has new-style classes.

Answer 2

You're initializing the rank and suit attributes without underscores but your property methods have _rank and _suit .

Also list.index throws an error if an item is not in the list. You should instead do

if value in valid_ranks: