Chanelling Guido - dict subclasses

[Mark Lawrence]

On 15/01/2014 01:27, Steven D'Aprano wrote:
> Over on the Python-Dev mailing list, there is an ENORMOUS multi-thread
> discussion involving at least two PEPs, about bytes/str compatibility.
> But I don't want to talk about that. (Oh gods, I *really* don't want to
> talk about that...)

+ trillions

>
> In the midst of that discussion, Guido van Rossum made a comment about
> subclassing dicts:
>
>      [quote]
>      From: Guido van Rossum <guido at python.org>
>      Date: Tue, 14 Jan 2014 12:06:32 -0800
>      Subject: Re: [Python-Dev] PEP 460 reboot
>
>      Personally I wouldn't add any words suggesting or referring
>      to the option of creation another class for this purpose. You
>      wouldn't recommend subclassing dict for constraining the
>      types of keys or values, would you?
>      [end quote]
>
> https://mail.python.org/pipermail/python-dev/2014-January/131537.html
>
> This surprises me, and rather than bother Python-Dev (where it will
> likely be lost in the noise, and certain will be off-topic), I'm hoping
> there may be someone here who is willing to attempt to channel GvR. I
> would have thought that subclassing dict for the purpose of constraining
> the type of keys or values would be precisely an excellent use of
> subclassing.

Exactly what I was thinking.

>
>
> class TextOnlyDict(dict):
>      def __setitem__(self, key, value):
>          if not isinstance(key, str):
>              raise TypeError
>          super().__setitem__(key, value)
>      # need to override more methods too
>
>
> 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?
>
>

Couple of replies I noted from Ned Batchelder and Terry Reedy.  Smacked 
bottom for Peter Otten, how dare he? :)

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence