[Python-Dev] PEP 246, redux

[Clark C. Evans]

On Tue, Jan 11, 2005 at 03:30:19PM -0500, Phillip J. Eby wrote:
| Clark said he didn't want to assume substitutability; I was pointing out 
| that he could choose to not assume that, if he wished, by implementing an 
| appropriate __conform__ at the base of his hierarchy. 

Oh, that's sufficient.  If someone making a base class wants to assert
that derived classes should check compliance (rather than having it
automagic), then they can do this.  Good enough!

| I don't agree with Clark's use case, but my 
| proposal supports it as a possibility, and yours does not.

It was a straw-man; and I admit, not a particularly compelling one.

| To implement a Liskov violation with my proposal, you do exactly the same 
| as with your proposal, *except* that you can simply return None instead 
| of raising an exception, and the logic for adapt() is more 
| straightforward.

I think I prefer just returning None rather than raising a
specific exception.  The semantics are different: None implies that
other adaptation mechanisms (like a registry) could be tried, while
LiskovException implies that processing halts and no further 
adaptation techniques are to be used.  In this case, None is 
the better choice for this particular case since it would enable
third-parties to register a wrapper.

Overall, I think both you and Alex are now proposing essentially
the same thing... no?

Best,

Clark

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Clark C. Evans                      Prometheus Research, LLC.
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