如何为内置字典类的子类定义默认字典键

Question

SubClass() is a sub-class of built-in Python dictionary:

class SubClass(dict):
    def __init__(self, *args, **kwargs):
        super(SubClass, self).__init__(*args, **kwargs)

Now with any dictionary variable I can declare an instance of SubClass . Each dictionary key will become a key of SubClassed instance automatically (which is pretty cool):

myDictionary={'city':'New York', 'year':2014}
instance=SubClass(myDictionary)
print instance['city']
print instance.keys(), instance.values(), instance.items()

I would like to define a set of default dictionary keys so each SubClass object (instance) gets them on declaration. I could:

class SubClass(dict):
    def __init__(self, *args, **kwargs):
        super(SubClass, self).__init__(*args, **kwargs)
        self['id']=None
        self['number']=None
        self['oneMoreDefaultKey']=None

But that takes a lot of typing and looks ugly. I rather would like to achieve the same with a minimum amount of code.

Answer 1

I don't think that defaultdict is what you want, since that gives you the same value for any missing key, rather than just the ones you specify. You're only talking about 3 lines of code either way, so it's tough to imagine it getting too much simpler. However, you can achieve what you're looking for a little bit more concisely like this:

class SubClass(dict):
    DEFAULT_KEYS = ['id', 'number', 'oneMoreDefaultKey']
    def __init__(self, *args, **kwargs):
        super(SubClass, self).__init__(*args, **kwargs)
        self.update({key: None for key in self.DEFAULT_KEYS})

Answer 2

If you want certain, specific keys to have default values, you'll need to list those keys somewhere. Calling self.update() does what your code does:

class SubClass(dict):
    def __init__(self, *args, **kwargs):
        super(SubClass, self).__init__(*args, **kwargs)
        self.update({
            'id':None,
            'number':None,
            'oneMoreDefaultKey':None})

instance = SubClass({'city':'New York', 'year':2014})
print instance['id'], instance['year']

If you want all keys to have default values, and all of the default values are the same, then you are better off using collections.defaultdict :

import collections
instance = collections.defaultdict(lambda : None, {'city':'New York', 'year':2014})
print instance['id'], instance['year']