do not pass empty vector to Tables.columntable (#3435)

NEWS.md

@@ -42,6 +42,9 @@
 * Ensure that `allunique(::AbstractDataFrame, ::Any)` always gets
   interpreted as test for uniqueness of rows in the first positional argument
   ([#3434](https://github.com/JuliaData/DataFrames.jl/issues/3434))
+* Make sure that an empty vector of `Any` or of `AbstractVector` is treated as having
+  no columns when a data frame is being processed with `combine`/`select`/`transform`.
+  ([#3435](https://github.com/JuliaData/DataFrames.jl/issues/3435))
 
 # DataFrames.jl v1.6.1 Release Notes
 

src/abstractdataframe/selection.jl

@@ -822,7 +822,12 @@ function select_transform!((nc,)::Ref{Any}, df::AbstractDataFrame, newdf::DataFr
             res = newres
         elseif !(res isa Union{AbstractDataFrame, NamedTuple, DataFrameRow, AbstractMatrix,
                                Tables.AbstractRow})
-            res = Tables.columntable(res)
+            if res isa Union{AbstractVector{Any}, AbstractVector{<:AbstractVector}}
+                @assert isempty(res)
+                res = DataFrame()
+            else
+                res = Tables.columntable(res)
+            end
         end
     end
 

test/select.jl

@@ -3024,4 +3024,22 @@ end
     @test_throws ArgumentError combine(gdf, :x => (x -> x[1] == 2 ? "x" : cr) => AsTable)
 end
 
+@testset "empty vector" begin
+    df = DataFrame(a=1:3)
+
+    @test_throws ArgumentError select(df, :a => (x -> Vector{Any}[]))
+
+    for T in (Vector{Any}, Any, NamedTuple{(:x,),Tuple{Int64}})
+        v = combine(df, :a => (x -> T[])).a_function
+        @test isempty(v)
+        @test eltype(v) === T
+    end
+
+    @test size(combine(df, :a => (x -> Vector{Any}[]) => AsTable)) == (0, 0)
+    @test size(combine(df, :a => (x -> Any[]) => AsTable)) == (0, 0)
+    df2 = combine(df, :a => (x -> NamedTuple{(:x,),Tuple{Int64}}[]) => AsTable)
+    @test size(df2) == (0, 1)
+    @test eltype(df2.x) === Int
+end
+
 end # module