Correct usage of a getter/setter for dictionary values

python getter-setter
jb. picture jb. · Oct 13, 2011 · Viewed 12.8k times · Source

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

I have an object with this dictionary:

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

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

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.

Answer

unutbu picture unutbu · Oct 13, 2011

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

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. 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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