Michael changed from set to list at my urging. A list is more general. A groupby in Pandas or SQL does not enforce uniqueness, but DOES preserve order. I think the PEP is not fully updated, but it's a list everywhere in the proposal itself, just not in the "old techniques."

Moreover,  Michael gives example of "casting" the Grouping to a dictionary with either sets or Counters as values. Both are useful, and both can be derived from list. But you cannot go backwards from either to the list. The transformation is simple and obvious, and can be included in eventual documentation.

It really is better to construct the collection using lists—in the fully general manner—and then only throw away the generality when that appropriate.

On Sun, Jul 1, 2018, 8:47 PM Chris Barker via Python-ideas <python-ideas@python.org> wrote:


Ivan,

Did you mean this to go to the list? I hope so, as I've cc-d it this time :-)

On Sun, Jul 1, 2018 at 1:20 AM, Ivan Levkivskyi <levkivskyi@gmail.com> wrote:
On 1 July 2018 at 06:18, Chris Barker via Python-ideas <python-ideas@python.org> wrote:
I'm really warming to the:

Alternate: collections.Grouping

version -- I really like this as a kind of custom mapping, rather than "just a function" (or alternate constructor) --

I wanted the group to be represented as a set, not a list. I however understand that list may be more common. Can we design an API that
would make this configurable? Something like:

    from collections import Grouping

    deps = Grouping(set)  # list can be the default
    deps.update(other_deps)  # uses set.update or list.extend for every key
    deps.add(trigger, target)  # uses set.add or list.append

yeah, I thought about that too -- Michael was using set() in some of his examples.

But the question is -- do we have a single switchable version or just too classes?
 
Probably allowing an arbitrary collection for values is to general/hard.

maybe not -- if we had the criteria that you pass in any collection you wanted, as long as it had either an .append() or .add()method, it would be pretty easy to do with duck typing magic.

Sure -- a user could make a mess easily enough by passing in a weird custom class, but so what? Using something other than a set or list would be a "at your own risk" thing anyway.
 
Maybe we can just add a flag `unique=True` to the constructor, that will cause using sets instead of lists for groups?

That's another, more robust, but less flexible option.

Stay tuned for a prototype, if I can get it done fast....

-CHB


--

Christopher Barker, Ph.D.
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--

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chris.Barker@noaa.gov
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