[Python-Dev] defaultdict proposal round three (original) (raw)

Guido van Rossum guido at python.org
Tue Feb 21 00:17:56 CET 2006

On 2/20/06, Brett Cannon <brett at python.org> wrote:

While #3 is my preferred solution as well, it does pose a Liskov violation if this is a direct dict subclass instead of storing a dict internally (can't remember the name of the design pattern that does this). But I think it is good to have the constructor be different since it does also help drive home the point that this is not a standard dict.

I've heard this argument a few times now from different folks and I'm tired of it. It's wrong. It's not true. It's a dead argument. It's pushing up the daisies, so to speak.

Please stop abusing Barbara Liskov's name and remember that the constructor signature is not part of the interface to an instance! Changing the constructor signature in a subclass does not cause any "Liskov" violations because the constructor is not called by users of the object -- it is only called to create an object. As the user of an object you're not allowed to create another instance (unless the object provides an explicit API to do so, of course, in which case you deal with that API's signature, not with the constructor).

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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