Python dictionary with default key

Question

I am running Python 2.7.10.

I would like to have a dictionary return the value stored at a particular key in case of missing item. For example, something like that:

myD = dict(...)
return myD[key] if key in myD else myD[defaultKey]

Just to make sure it is clear, I want to call myD[key] and have the right value returned without the extra if...else in my code...

This isn't quite what defaultdict does (since it takes a function to call as a default) and not quite what dict.setdefault() does, and myD.get(key, ???) does not seem to help either. I probably should inherit from dict or defaultdict and overload __init__() and missing() methods, but I could not come up with a good way to do this.

Answer 1

I'm not completely sure what you want (didn't read all the comments under your question), but think this may be at least close to what you want.

class DefaultKeyDict(dict):
    def __init__(self, default_key, *args, **kwargs):
        self.default_key = default_key
        super(DefaultKeyDict, self).__init__(*args, **kwargs)

    def __missing__ (self, key):
        if self.default_key not in self:  # default key not defined
            raise KeyError(key)
        return self[self.default_key]

    def __repr__(self):
        return ('{}({!r}, {})'.format(self.__class__.__name__,
                                      self.default_key,
                                      super(DefaultKeyDict, self).__repr__()))

    def __reduce__(self):  # optional, for pickle support
        args = (self.default_key if self.default_key in self else None,)
        return self.__class__, args, None, None, self.iteritems()

dkdict = DefaultKeyDict('b', {'a': 1, 'b': 2, 'c': 3})

print dkdict['a']  # -> 1
print dkdict['b']  # -> 2
print dkdict['c']  # -> 3
print dkdict['d']  # -> 2 (value of the default key)

del dkdict['b']  # delete the default key
try:
    print dkdict['d']  # should now raise exception like normal
except KeyError:
    print("dkdict['d'] raised a KeyError")

You might want to modify the class __init__() method to accept both the default key and its value as arguments (instead of just the key).