[Python-ideas] Add orderedset as set(iterable, *, ordered=False) and similarly for frozenset.

[Ed Kellett]

On Sun Feb 08 2015 at 12:52:56 AM Chris Angelico <rosuav at gmail.com> wrote:

> To be quite honest, I would be very happy if sets had more of the
> operations that dictionaries have. Imagine if "set" were "dictionary
> where all the values are True", and "OrderedSet" were "OrderedDict
> where all the values are True"; combining sets is like combining
> dictionaries, but you don't have to worry about merging the values.
>

I think they already have most of the useful ones. Adding True values seems
unnecessary, to me—that entries have no value seems more appropriate in
terms of merging them making sense.


> ISTM ordered set addition/union should be like list concatenation with
> duplicates dropped.


I agree about addition - that'd mirror list concatenation nicely. But with
that in mind, making union do the same thing seems wrong - partly because
it'd be an exact duplicate of other functionality, and partly because it's
a special case:

- `set | set` has no meaningful ordering
- `orderedset | set` has no meaningful ordering
- `set | orderedset` has no meaningful ordering
- `orderedset | orderedset` has a meaningful ordering if you expect union
to act like update(). Maybe that's more common than I thought; I always
considered them fundamentally different concepts.

Ed Kellett
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