[Python-ideas] Sorted lists

Steven D'Aprano steve at pearwood.info
Mon Apr 8 05:17:30 EDT 2019

On Mon, Apr 08, 2019 at 01:31:14PM +1000, Cameron Simpson wrote:

> __setitem__ concerns me, along with other modification methods: what 
> about subclasses(*)? Every existing subclass which overrides __setitem__ 
> now needs to grow code to maintain __issorted__ if they do not 
> themselves call list.__setitem__.

Well, yeah. That's what happens if you subclass: you are responsible for 
maintaining the invariant if you don't call the superclasses.

Maybe there's a cunning way to avoid this, but that will make the 
implementation more complex and probably tips this proposal from "maybe 
worth thinking about" to "not a chance".

But I'd be inclined to just pass the buck to the subclass: if you want 
to maintain the invariant, then you have to maintain it, or call the 
appropriate super methods.

Now that you've reminded me of subclasses, I would make one other change 
to the specs: all lists, including empty ones, are created with the flag 
set to False. That way subclasses which *don't* maintain the invariant 
will always be flagged as unsorted (unless the caller explicitly sets 
the flag themselves).

> Also, should this imply an issorted() builtin to consult an instance's 
> __issorted__ dunder flag? Should such a builtin return False for 
> instances without an __issorted__ flag? I'm thinking yes since the flag 
> is intended to mean known-to-be-sorted.

I don't think this proposal is worth adding a builtin function. Not 
unless somebody thinks of some more very compelling use-cases.

Perhaps an inspect.sort_hint() function.


More information about the Python-ideas mailing list