PEP 245

[Clark C. Evans]

On Fri, 6 Apr 2001, Scott Finnie wrote:
> Hmmm.  A question about PEP245: how do interfaces differ from types?

I'm not part of the core team, but here is my understanding:

1) "type" is a function, i.e., type(thing) returns one
   and only one result, type(myObject) is types.InstanceType

2) "instance" is a relation, i.e., isinstance(object,interface)
   gives a boolean telling you if (object,interface) are a 
   member of this relation.  

3) Types are used internally to determine how they are 
   treated, all instance types are treated in a similar way
   where as a built-in types have a different C/Java 
   implementation.

4) Interfaces will be user definable, types are not.

5) To change types would be difficult (due to backward compatibility)

6) I'd like to have interfaces soon, and rather not wait
   for the type system to be complete re-written.

> There are currently limitations on typing (only covers built-ins, object
> can only be of a single type) but neither of these seem justifiable
> reasons for introducing a new construct rather than extending the
> existing one?

I'm sure Guido will take this position into account when
examining the PEP.  

;) Clark