Chanelling Guido - dict subclasses

[Daniel da Silva]

On Tue, Jan 14, 2014 at 8:27 PM, Steven D'Aprano <
steve+comp.lang.python at pearwood.info> wrote:
>
> But reading Guido, I think he's saying that wouldn't be a good idea. I
> don't get it -- it's not a violation of the Liskov Substitution
> Principle, because it's more restrictive, not less. What am I missing?
>

Just to be pedantic, this *is* a violation of the Liskov Substution
Principle. According to Wikipedia, the principle states:

 if S is a subtype <http://en.wikipedia.org/wiki/Subtype> of T, then
> objects of type <http://en.wikipedia.org/wiki/Datatype> T may be replaced
> with objects of type S (i.e., objects of type S may be *substituted* for
> objects of type T) without altering any of the desirable properties of that
> program (correctness, task performed, etc.) [0]<http://en.wikipedia.org/wiki/Liskov_substitution_principle>


 Since S (TextOnlyDict) is more restrictive, it cannot be replaced for T
(dict) because the program may be using non-string keys.


Daniel
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