[Python-3000] PEP: Information Attributes

[Josiah Carlson]

"Raymond Hettinger" <python at rcn.com> wrote:
> Proto-PEP:  Information Attributes (First Draft)
> 
> Proposal:
> 
> Testing hasattr() is a broadly applicable and flexible technique that works well
> whenever the presence of a method has an unambiguous interpretation
>  (i.e. __hash__ for hashability, __iter__ for iterability, __len__ for sized
> containers); however, there are other methods with ambiguous interpretations
> that could be resolved by adding an information attribute.

To me, this seems more like traits/roles than ABCs.  Though I haven't
weighed in on either of them, generally I'm with Raymond and others in
the whole "ABCs seem like overkill" perspective.  As such, I'm -1 on
ABCs, but +1 on the general idea of traits/roles - of which I would
consider this PEP to be one.

My concern with Information Attributes is similar to my concern about
ABCs; in order to state the information available from these information
attributes, they need to be part of the class or instance.  On built-in
types, users would not be able to add things to classes or instances, as
is the case with the numpy folks wanting to add 'ring' to integers.

While I've not seen a PEP for offering live traits/roles addition or
removal, I suspect that it would involve weak key dictionaries adding
traits to classes, and only allow hashable instances for single-object
trait additions (depending on the kinds of traits/roles, it could
probably be implemented as a dictionary of weak key sets). I would be +1
in this case, as it would offer most of the benefits of ABCs*, with none
of the pre-implementation drawbacks.


 - Josiah

* Related to ABCs is the __issubclass__ and __isinstance__ stuff that
allows for proxy objects directly.  Traits/roles could be massaged to do
similar things, but using __is...__ directly seems like it would perform
this operation better.  I'm not a real big