Please help about an instance var

[Gabriel Genellina]

En Tue, 13 Feb 2007 23:54:29 -0300, <JStoneGT at aol.com> escribió:

>>>> from UserDict import UserDict
>>>> d =  {1:2,3:4,5:6}
>>>> d1 = UserDict(d)
>>>> d1
> {1: 2, 3:  4, 5: 6}
>>>> d1.data
> {1: 2, 3: 4, 5: 6}
> Here why both d1 and d1.data have the same values?As shown below,they're
> different types.
>
>>>> type(d1)
> <type 'instance'>
>>>>  type(d1.data)
> <type 'dict'>
> Please help.Thanks!

UserDict is a specially crafted class that looks and acts like a  
dictionary, so, its str() and repr() looks like a true dictionary.
But as you have seen, using type() you can distinguish between the two.  
d1.__class__ is different too, as many other details; but many functions  
don't require a "true" dictionary anyway, just use methods like keys(),  
items(), and magic support for d[key], d[key]=value, and so on.
As far as UserDict provides those methods, all functions expecting a  
dictionary-like argument are happy.

An historical note: On older Python versions, you could not inherit from  
builtin types. If you wanted a custom dictionary-like class, you had to  
inherit from UserDict or a similar class.
Now, you can inherit from dict if you want; and DictMixin can be used to  
provide any class with dictionary functionalities in a much convenient way.

-- 
Gabriel Genellina