Adding an attribute to a Python dictionary from the standard library

I was wondering if you could add an attribute to a Python dictionary?

class myclass():
    def __init__(): 
     self.mydict = {}  # initialize a regular dict
     self.mydict.newattribute = "A description of what this dictionary will hold"
     >>> AttributeError: 'dict' object has no attribute 'newattribute'
     setattr(self.mydict, "attribute", "A description of what this dictionary will hold"
     >>> AttributeError: 'dict' object has no attribute 'newattribute'

Is there anyway to quickly add my description attribute without having to copy the dict class and overloading the constructor. I thought it would be simple, but I guess I was wrong.

Answer

Just derive from dict:

class MyDict(dict):
    pass

Instances of MyDict behave like a dict, but can have custom attributes:

>>> d = MyDict()
>>> d.my_attr = "whatever"
>>> d.my_attr
'whatever'