[Python-Dev] PEP 246: lossless and stateless

[Phillip J. Eby]

At 10:09 AM 1/14/05 +0100, Just van Rossum wrote:
>Guido van Rossum wrote:
>
> > Are there real-life uses of stateful adapters that would be thrown out
> > by this requirement?
>
>Here are two interfaces we're using in a project:
>
>   http://just.letterror.com/ltrwiki/PenProtocol (aka "SegmentPen")
>   http://just.letterror.com/ltrwiki/PointPen
>
>They're both abstractions for drawing glyphs (characters from a font).
>Sometimes the former is more practical and sometimes the latter. We
>really need both interfaces. Yet they can't be adapted without keeping
>some state in the adapter.

Maybe I'm missing something, but for those interfaces, isn't it okay to 
keep the state in the *adapted* object here?  In other words, if PointPen 
just added some private attributes to store the extra data?


>Implicit adaptations may be dangerous here, but I'm not so sure I care.
>In my particular use case, it will be very rare that people want to do
>
>     funcTakingPointPen(segmentPen)
>     otherFuncTakingPointPen(segmentPen)

But if the extra state were stored on the segmentPen rather than the 
adapter, this would work correctly, wouldn't it?  Whereas with it stored in 
an adapter, it wouldn't.