dictionary - A python class that acts like dict

Question

I want to write a custom class that behaves like dict - so, I am inheriting from dict.

My question, though, is: Do I need to create a private dict member in my __init__() method?. I don't see the point of this, since I already have the dict behavior if I simply inherit from dict.

Can anyone point out why most of the inheritance snippets look like the one below?

class CustomDictOne(dict):
   def __init__(self):
      self._mydict = {} 

   # other methods follow

Instead of the simpler...

class CustomDictTwo(dict):
   def __init__(self):
      # initialize my other stuff here ...

   # other methods follow

Actually, I think I suspect the answer to the question is so that users cannot directly access your dictionary (i.e. they have to use the access methods that you have provided).

However, what about the array access operator []? How would one implement that? So far, I have not seen an example that shows how to override the [] operator.

So if a [] access function is not provided in the custom class, the inherited base methods will be operating on a different dictionary?

I tried the following snippet to test out my understanding of Python inheritance:

class myDict(dict):
    def __init__(self):
        self._dict = {}

    def add(self, id, val):
        self._dict[id] = val


md = myDict()
md.add('id', 123)
print md[id]

I got the following error:

KeyError: < built-in function id>

What is wrong with the code above?

How do I correct the class myDict so that I can write code like this?

md = myDict()
md['id'] = 123

[Edit]

I have edited the code sample above to get rid of the silly error I made before I dashed away from my desk. It was a typo (I should have spotted it from the error message).

Answer 1

class Mapping(dict):

    def __setitem__(self, key, item):
        self.__dict__[key] = item

    def __getitem__(self, key):
        return self.__dict__[key]

    def __repr__(self):
        return repr(self.__dict__)

    def __len__(self):
        return len(self.__dict__)

    def __delitem__(self, key):
        del self.__dict__[key]

    def clear(self):
        return self.__dict__.clear()

    def copy(self):
        return self.__dict__.copy()

    def has_key(self, k):
        return k in self.__dict__

    def update(self, *args, **kwargs):
        return self.__dict__.update(*args, **kwargs)

    def keys(self):
        return self.__dict__.keys()

    def values(self):
        return self.__dict__.values()

    def items(self):
        return self.__dict__.items()

    def pop(self, *args):
        return self.__dict__.pop(*args)

    def __cmp__(self, dict_):
        return self.__cmp__(self.__dict__, dict_)

    def __contains__(self, item):
        return item in self.__dict__

    def __iter__(self):
        return iter(self.__dict__)

    def __unicode__(self):
        return unicode(repr(self.__dict__))


o = Mapping()
o.foo = "bar"
o['lumberjack'] = 'foo'
o.update({'a': 'b'}, c=44)
print 'lumberjack' in o
print o

In [187]: run mapping.py
True
{'a': 'b', 'lumberjack': 'foo', 'foo': 'bar', 'c': 44}