[Feature Request] Multiple values from NamedTuple

[kescobo]

I think it would be useful to have some syntax for returning a subset of a NamedTuple as another NamedTuple, analogous to the same ability with regular Tuples.

julia> tup = (1, 2, 3)
(1, 2, 3)

julia> tup[1]
1

julia> tup[[1,2]]
(1, 2)

julia> tup[[1]]
(1,)

julia> nt = (a=1, b=2, c=3)
(a = 1, b = 2, c = 3)

julia> nt[:a]
1

julia> nt[[:a, :b]]
ERROR: MethodError: no method matching getindex(::NamedTuple{(:a, :b, :c), Tuple{Int64, Int64, Int64}}, ::Vector{Symbol})

The following works, though I don't know if it's the most efficient:

julia> get(n::NamedTuple, ntkeys) = (; (s=>n[s] for s in ntkeys)...)
get (generic function with 2 methods)

julia> get(nt, [:a,:b])
(a = 1, b = 2)

julia> get(nt, [:a])
(a = 1,)

I'm not advocating it actually be called get - I think something that worked with getindex would be ideal. NamedTupleTools.jl has a method called select that does this, but it seems like this functionality would be great to have in Base.

One counter argument is that this doesn't work with Dicts

julia> dict = Dict(pairs(nt)...)
Dict{Symbol, Int64} with 3 entries:
  :a => 1
  :b => 2
  :c => 3

julia> dict[[:a, :b]]
ERROR: KeyError: key [:a, :b] not found

But it is technically possible to have an array as a key in a Dict, but not for a NamedTuple.

I'm happy to make a PR if this idea is acceptable, and if there aren't any implications that are escaping me.

[kescobo]

Another suggestions from slack helpdesk:

julia> sel = (:a, :b);

julia> NamedTuple{sel}((nt[k] for k in sel))
(a = 1, b = 2)

[JeffBezanson]

The correct way to do it currently is

julia> nt = (a=1, b=2, c=3)

julia> NamedTuple{(:a, :b)}(nt)
(a = 1, b = 2)

i.e. the generator there is just redundant.

We could maybe define

@inline getindex(nt::NamedTuple, s::Tuple{Vararg{Symbol}}) = NamedTuple{s}(nt)

though this also conflicts with Dicts unfortunately.

[kescobo]

though this also conflicts with Dicts unfortunately.

Do you mean conceptually or literally? Dicts aren't a subtype of NamedTuple, right?

NamedTuple{(:a, :b)}(nt) is a little clunky, but if adding the getindex method is problematic, perhaps just making it clearer in the docs so it's easier to find. I had looked here, here, and the docstring for NamedTuple. It turns out that it is in the docstring

NamedTuple{names}(nt::NamedTuple)

  Construct a named tuple by selecting fields in names (a tuple of Symbols) from another named tuple.

but I guess because this signature is so dissimilar from the types of selection syntax I'm used to, I skimmed past it.

[JeffBezanson]

I guess we could add the getindex method.

[JeffreySarnoff]

though this also conflicts with Dicts unfortunately.

afaik The conflict is limited to Dicts with key[s] that are typed NTuple{N,Symbol}.
NamedTuples cannot have fieldnames that are typed NTuple{N,Symbol}.

I do not find the conflict all that concerning. If the desire is for conflict-free getindexing, there are two approaches that occur to me. Introduce getindicies that would require key[s] to be given as a Tuple, which would disambiguate the case of a key that is a tuple. Or use the dotted form of getindex which also presents the key[s] contained in a tuple.

[JeffBezanson]

duplicate of #27021