正确使用字典的getter/setter方法 [英] Correct usage of a getter/setter for dictionary values

问题描述

我对Python还是很陌生，所以如果这里有什么不好的事情，请指出.

I'm pretty new to Python, so if there's anything here that's flat-out bad, please point it out.

我有一个带有此字典的对象:

I have an object with this dictionary:

traits = {'happy': 0, 'worker': 0, 'honest': 0}

每个特征的值应为1到10的整数，并且不应添加新的特征.我要使用getter/setter，所以我可以确保保留这些约束.这是我现在制作吸气剂和吸气剂的方法:

The value for each trait should be an int in the range 1-10, and new traits should not be allowed to be added. I want getter/setters so I can make sure these constraints are being kept. Here's how I made the getter and setter now:

def getTrait(self, key):
    if key not in self.traits.keys():
        raise KeyError

    return traits[key]

def setTrait(self, key, value):
    if key not in self.traits.keys():
        raise KeyError

    value = int(value)

    if value < 1 or value > 10:
        raise ValueError

    traits[key] = value

我在此网站上阅读了有关property()方法的信息.但是我没有找到一种简单的方法来利用它来获取/设置字典中的值.有一个更好的方法吗?理想情况下，我希望此对象的用法为obj.traits['happy'] = 14，它将调用我的setter方法并由于14超过10而抛出ValueError.

I read on this website about the property() method. But I don't see an easy way to make use of it for getting/setting the values inside the dictionary. Is there a better way to do this? Ideally I would like the usage of this object to be obj.traits['happy'] = 14, which would invoke my setter method and throw a ValueError since 14 is over 10.

推荐答案

如果您愿意使用obj['happy'] = 14这样的语法，则可以使用__getitem__和__setitem__:

If you are willing to use syntax like obj['happy'] = 14 then you could use __getitem__ and __setitem__:

def __getitem__(self, key):
    if key not in self.traits.keys():
        raise KeyError
    ... 
    return traits[key]

def __setitem__(self, key, value):
    if key not in self.traits.keys():
        raise KeyError
    ...
    traits[key] = value

如果您确实想要obj.traits['happy'] = 14，则可以定义dict的子类，并将obj.traits用作此子类的实例. 然后，子类将覆盖__getitem__和__setitem__(请参见下文).

If you really do want obj.traits['happy'] = 14 then you could define a subclass of dict and make obj.traits an instance of this subclass. The subclass would then override __getitem__ and __setitem__ (see below).

PS.要继承dict的子类，请同时继承collections.MutableMapping和dict.否则，dict.update不会调用新的__setitem__.

PS. To subclass dict, inherit from both collections.MutableMapping, and dict. Otherwise, dict.update would not call the new __setitem__.

import collections
class TraitsDict(collections.MutableMapping,dict):
    def __getitem__(self,key):
        return dict.__getitem__(self,key)
    def __setitem__(self, key, value):
        value = int(value)
        if not 1 <= value <= 10:
            raise ValueError('{v} not in range [1,10]'.format(v=value))
        dict.__setitem__(self,key,value)
    def __delitem__(self, key):
        dict.__delitem__(self,key)
    def __iter__(self):
        return dict.__iter__(self)
    def __len__(self):
        return dict.__len__(self)
    def __contains__(self, x):
        return dict.__contains__(self,x)

class Person(object):
    def __init__(self):
        self.traits=TraitsDict({'happy': 0, 'worker': 0, 'honest': 0})

p=Person()
print(p.traits['happy'])
# 0

p.traits['happy']=1
print(p.traits['happy'])
# 1

p.traits['happy']=14
# ValueError: 14 not in range [1,10]

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