[Python-ideas] OrderedCounter and OrderedDefaultDict

[Sven R. Kunze]

On 21.10.2015 05:36, Guido van Rossum wrote:
> Right. Chris's thinking recalls the reason why inheritance at some 
> became popular (I guess it was in the '90s). Steven explains (in the 
> part that I've cut) why many experts have soured on it quite a bit.
>

Yep. It's because experience shows that the usefulness is limited when 
it comes to orthogonal aspects.

Furthermore, I can remember changing an algorithm's design **because** 
of the lack of such built-in datastructure. That's always a bad sign.

> Personally, I happen to think that inheritance is often useful when a 
> number of classes are designed together (typically all at once and 
> belonging to the same library) and also occasionally when a base class 
> is explicitly and carefully designed to be inherited (there are 
> beautiful things you can do with the template pattern, for example). 
> But inheriting from an implementation that wasn't designed with your 
> use case in mind is often asking for trouble -- if not now, then in 
> the future when a refactored implementation is released.

That's a pretty good observation.

> You might interject, that's the fault of the implementation 
> refactoring -- they didn't properly think about interface 
> compatibility. But while it's usually easy enough to keep an interface 
> compatible where it's just the user calling methods on the 
> implementation, the "interface" presented by subclassing is much more 
> complex -- you would have to specify exactly which method's 
> implementation calls which other method, and you'd also have to ensure 
> that the object is in a sane state when it calls that other method, 
> because it *could* be the case that the latter is overridden by a 
> subclass. It's terribly fragile, and better avoided.

Maybe, that's one reason why people hesitate to write their own 
OrderDefaultDict or DefaultOrderedDict. It's just not their responsibility.

Best,
Sven