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 priva

Answer 1

Like this

class CustomDictOne(dict):
   def __init__(self,*arg,**kw):
      super(CustomDictOne, self).__init__(*arg, **kw)

Now you can use the built-in functions, like dict.get() as self.get().

You do not need to wrap a hidden self._dict. Your class already is a dict.

Answer 2

Here is an alternative solution:

class AttrDict(dict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__dict__ = self

a = AttrDict()
a.a = 1
a.b = 2

Answer 3

Check the documentation on emulating container types. In your case, the first parameter to add should be self.

Answer 4

The problem with this chunk of code:

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

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


md = myDict()
md.add('id', 123)

...is that your 'add' method (...and any method you want to be a member of a class) needs to have an explicit 'self' declared as its first argument, like:

def add(self, 'id', 23):

To implement the operator overloading to access items by key, look in the docs for the magic methods __getitem__ and __setitem__.

Note that because Python uses Duck Typing, there may actually be no reason to derive your custom dict class from the language's dict class -- without knowing more about what you're trying to do (e.g, if you need to pass an instance of this class into some code someplace that will break unless isinstance(MyDict(), dict) == True), you may be better off just implementing the API that makes your class sufficiently dict-like and stopping there.

Answer 5

A python class that acts like dict

By request - adding slots to a dict subclass.

Why add slots? A builtin dict instance doesn't have arbitrary attributes:

>>> d = dict()
>>> d.foo = 'bar'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'dict' object has no attribute 'foo'

So, let's create a subclass the way most are doing it here on this answer:

my_dict(dict):
    """my subclass of dict""" 

md = my_dict()
md.foo = 'bar'

Since there's no error created by the above, the above class doesn't actually act, "like dict."

We can make it act like dict by giving it empty slots:

class my_dict(dict):
    __slots__ = ()

md = my_dict()

So now attempting to use arbitrary attributes will fail:

>>> md.foo = 'bar'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'my_dict' object has no attribute 'foo'

And this Python class acts more like a dict.

For more on how and why to use slots, see this Q&A: Usage of __slots__?

Answer 6

This is my best solution. I used this many times.

class DictLikeClass:
    ...
    def __getitem__(self, key):
        return getattr(self, key)

    def __setitem__(self, key, value):
        setattr(self, key, value)
    ...

You can use like:

>>> d = DictLikeClass()
>>> d["key"] = "value"
>>> print(d["key"])