On Wed, Feb 27, 2019 at 11:18 PM Serhiy Storchaka <storchaka@gmail.com> wrote:
27.02.19 20:48, Guido van Rossum пише:
> On Wed, Feb 27, 2019 at 10:42 AM Michael Selik
> <mike@selik.org
> <mailto:mike@selik.org>> wrote >     The dict subclass collections.Counter overrides the update method
>     for adding values instead of overwriting values.
>     https://docs.python.org/3/library/collections.html#collections.Counter.update
>     Counter also uses +/__add__ for a similar behavior.
>          >>> c = Counter(a=3, b=1)
>          >>> d = Counter(a=1, b=2)
>          >>> c + d # add two counters together:  c[x] + d[x]
>          Counter({'a': 4, 'b': 3})
>     At first I worried that changing base dict would cause confusion for
>     the subclass, but Counter seems to share the idea that update and +
>     are synonyms.
> Great, this sounds like a good argument for + over |. The other argument
> is that | for sets *is* symmetrical, while + is used for other
> collections where it's not symmetrical. So it sounds like + is a winner
> here.

Counter uses + for a *different* behavior!

 >>> Counter(a=2) + Counter(a=3)
Counter({'a': 5})

Well, you can see this as a special case. The proposed + operator on Mappings returns a new Mapping whose keys are the union of the keys of the two arguments; the value is the single value for a key that occurs in only one of the arguments, and *somehow* combined for a key that's in both. The way of combining keys is up to the type of Mapping. For dict, the second value wins (not so different as {'a': 1, 'a': 2}, which becomes {'a': 2}). But for other Mappings, the combination can be done differently -- and Counter chooses to add the two values.
I do not understand why we discuss a new syntax for dict merging if we
already have a syntax for dict merging: {**d1, **d2} (which works with
*all* mappings). Is not this contradicts the Zen?

But (as someone else pointed out) {**d1, **d2} always returns a dict, not the type of d1 and d2.

Also, I'm sorry for PEP 448, but even if you know about **d in simpler contexts, if you were to ask a typical Python user how to combine two dicts into a new one, I doubt many people would think of {**d1, **d2}. I know I myself had forgotten about it when this thread started! If you were to ask a newbie who has learned a few things (e.g. sequence concatenation) they would much more likely guess d1+d2.

The argument for + over | has been mentioned elsewhere already.

@Eric Smith
> I'd help out.

Please do! I tried to volunteer Stephen d'Aprano but I think he isn't interested in pushing through a controversial PEP.

The PEP should probably also propose d1-d2.

--Guido van Rossum (python.org/~guido)