Re: Chanelling Guido - dict subclasses

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Date	Wed, 15 Jan 2014 19:50:53 -0500
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Subject	Re: Chanelling Guido - dict subclasses
From	Daniel da Silva <var.mail.daniel@gmail.com>
To	"Steven D'Aprano" <steve+comp.lang.python@pearwood.info>
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On Tue, Jan 14, 2014 at 8:27 PM, Steven D'Aprano <
steve+comp.lang.python@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

Thread

Chanelling Guido - dict subclasses Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-01-15 01:27 +0000
  Re: Chanelling Guido - dict subclasses Ned Batchelder <ned@nedbatchelder.com> - 2014-01-14 21:04 -0500
  Re: Chanelling Guido - dict subclasses Terry Reedy <tjreedy@udel.edu> - 2014-01-14 22:48 -0500
  Re: Chanelling Guido - dict subclasses F <f@hop2it.be> - 2014-01-15 07:00 +0000
  Re: Chanelling Guido - dict subclasses Peter Otten <__peter__@web.de> - 2014-01-15 09:40 +0100
    Re: Chanelling Guido - dict subclasses John Ladasky <john_ladasky@sbcglobal.net> - 2014-01-15 08:51 -0800
      Re: Chanelling Guido - dict subclasses Peter Otten <__peter__@web.de> - 2014-01-15 19:35 +0100
      Re: Chanelling Guido - dict subclasses Devin Jeanpierre <jeanpierreda@gmail.com> - 2014-01-15 22:30 -0800
  Re: Chanelling Guido - dict subclasses Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-01-15 09:10 +0000
  Re: Chanelling Guido - dict subclasses Tim Chase <python.list@tim.thechases.com> - 2014-01-15 05:03 -0600
  Re: Chanelling Guido - dict subclasses Daniel da Silva <var.mail.daniel@gmail.com> - 2014-01-15 19:50 -0500
    Re: Chanelling Guido - dict subclasses Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-01-16 17:17 +1300

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