[Python-Dev] Python 3.6 dict becomes compact and gets a private version; and keywords become ordered

[Tim Delaney]

On 15 September 2016 at 05:02, Terry Reedy <tjreedy at udel.edu> wrote:

>
> We already have compact mutable collection types that can be kept
> insert-ordered if one chooses -- lists and collections.deque -- and they
> are not limited to hashables.  Before sets were added, either lists or
> dicts with None values were used as sets.  The latter is obsolete but lists
> are still sometimes used for their generality, as in a set of lists.  We
> now also have enums for certain small frozensets where the set opertions
> are not needed.


One use case that isn't covered by any of the above is removing duplicates
whilst retaining order (of the first of the matching elements). With an
OrderedSet (or ordered by default sets) it would be as simple as:

a = OrderedSet(iterable)

Probably the best current option would be:

a = list(OrderedDict(k, None for k in iterable))

The other use I have for an ordered set is to build up an iterable of
unique values whilst retaining order. It's a lot more efficient than doing
a linear search on a list when adding each element to see if it's already
present.

In many cases the order is primarily important for debugging purposes, but
I can definitely find cases in my current java codebase where I've used the
pattern (LinkedHashSet) and the order is important to the semantics of the
code.

Tim Delaney
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