[Python-Dev] PEP 246, redux (original) (raw)

Phillip J. Eby pje at telecommunity.com
Mon Jan 10 19:34:59 CET 2005

At 01:19 PM 1/10/05 -0500, Clark C. Evans wrote:

Alex,

This is wonderful work, thank you for keeping the ball in the air; I'm honored to keep my name as a co-author -- kinda like a free lunch. Phillip, Once again, thank you! Without PyProtocols and your advocacy, this proposal might have been buried in the historical bit-bucket. On Mon, Jan 10, 2005 at 12:43:44PM -0500, Phillip J. Eby wrote: | -1 if this introduces a performance penalty to a wide range of | adaptations (i.e. those using abstract base classes), just to support | people who want to create deliberate Liskov violations. I personally | don't think that we should pander to Liskov violators, especially since | Guido seems to be saying that there will be some kind of interface | objects available in future Pythons. I particularly like Alex's Liskov violation error; although it is not hugely common, it does happen, and there should be a way for a class to indicate that it's only being used for implementation. Perhaps... if the class doesn't have a conform method, then its adaptation is automatic (that is, only the class can raise this case). The rationale for only enabling one of the two paths is that the base class would have been in-place before the derived class was created; therefore, it is highly unlikely that adapt would ever be of help. Therefore, there might be a performance penalty, but it'd be really small, simply checking to see if the slot is filled in.

The performance penalty I was talking about was for using an abstract base class, in a subclass with a conform method for conformance to other protocols. In this case, conform will be uselessly called every time the object is adapted to the abstract base class.

IMO it's more desirable to support abstract base classes than to allow classes to "opt out" of inheritance when testing conformance to a base class. If you don't have an "is-a" relationship to your base class, you should be using delegation, not inheritance. (E.g. 'set' has-a 'dict', not 'set' is-a 'dict', so 'adapt(set,dict)' should fail, at least on the basis of isinstance checking.)

The other problem with a Liskov opt-out is that you have to explicitly do a fair amount of work to create a LiskovViolation-raising subclass; that work would be better spent migrating to use delegation instead of inheritance, which would also be cleaner and more comprehensible code than writing a conform hack to announce your bad style in having chosen to use inheritance where delegation is more appropriate. ;)

This latter problem is actually much worse than the performance issue, which was just my initial impression. Now that I've thought about it some more, I think I'm against supporting Liskov violations even if it were somehow faster. :)

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