[Python-Dev] UserDict in 2.7

[Michael Foord]

On 23/06/2010 00:03, Greg Ewing wrote:
> Benjamin Peterson wrote:
>
>> IIRC this was because UserDict tries to be a MutableMapping but abcs
>> require new style classes.
>
> Are there any use cases for UserList and UserDict in new
> code, now that list and dict can be subclassed?

Inheriting from list or dict isn't very useful as you to have to 
override *every* method to control behaviour.

(For example with the dict if you override __setitem__ then update and 
setdefault (etc) don't go through your new __setitem__ and if you 
override __getitem__ then pop and friends don't go through your new 
__getitem__.)

In 2.6+ you can of course use the collections.MutableMapping abc, but if 
you want to write cross-Python version code UserDict is still useful. If 
you want abc support then you are *already* on 2.6+ though I guess.

All the best,

Michael

>
> If not, I don't think it would be a big problem if they
> were left out of the ABC ecosystem. No worse than what
> happens to any other existing user-defined class that
> predates ABCs -- if people want them to inherit from
> ABCs, they have to update their code. In this case, the
> update would consist of changing subclasses to inherit
> from list or dict instead.
>


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