I want to subclass dict and set default values

问题

I have a need to create a special subclass of dict. In it I want to set default values for a set of keys.

I seem to be failing in finding the correct syntax to do this.

Here is what I have been trying:

class NewDict(dict):
    Key1 = "stuff"
    Key2 = "Other stuff"
    NoList = []
    Nada = None

I am then instantiating an object like this:

PrefilledDict = NewDict()

and trying to use something in there:

print PrefilledDict['Key1']

But it seems that my dictionary is not a dictionary.

What little bit am I missing?

回答1:

You can achieve what you want as such:

class NewDict(dict):

    def __init__(self):
        self['Key1'] = 'stuff'
        ...

PrefilledDict = NewDict()
print PrefilledDict['Key1']

With your code, you are creating attributes of the NewDict class, not keys in the dictionary, meaning that you would access the attributes as such:

PrefilledDict = NewDict()
print PrefilledDict.Key1

回答2:

No subclassing needed:

def predefined_dict(**kwargs):
    d = {
        'key1': 'stuff',
        ...
    }
    d.update(kwargs)
    return d

new_dict = predefined_dict()
print new_dict['key1']

or just:

defaults = {'a':1, 'b':2}
new_dict = defaults.copy()
print new_dict['a']

回答3:

@astynax provided a good answer but if you must use a subclass you could:

class defaultattrdict(dict):
    def __missing__(self, key):
        try: return getattr(self, key)
        except AttributeError:
            raise KeyError(key) #PEP409 from None

Then:

class NewDict(defaultattrdict):
    Key1 = "stuff"
    Key2 = "Other stuff"
    NoList = []
    Nada = None

PrefilledDict = NewDict()
print(PrefilledDict['Key1']) # -> "stuff"
print(PrefilledDict.get('Key1')) #NOTE: None as defaultdict

Note: your code doesn't follow pep8 naming convention.

来源：https://stackoverflow.com/questions/10901048/i-want-to-subclass-dict-and-set-default-values

标签

python

dictionary

subclass