fix nonunique bug (#3393)

Author: bkamins

NEWS.md

@@ -20,6 +20,9 @@
   instead of using the interactive thread pool when Julia was started
   with `-tM,N` with N > 0
   ([#3385](https://github.com/JuliaData/DataFrames.jl/pull/3385))
+* Correctly return `Bool[]` in the `nonunique` function applied to a data frame
+  with a pulled column that has zero levels in the pool
+  ([#3393](https://github.com/JuliaData/DataFrames.jl/pull/3393))
 
 # DataFrames.jl v1.6.1 Release Notes
 

src/abstractdataframe/unique.jl

@@ -87,7 +87,7 @@ function nonunique(df::AbstractDataFrame; keep::Symbol=:first)
     if !(keep in (:first, :last, :noduplicates))
         throw(ArgumentError("`keep` must be :first, :last, or :noduplicates"))
     end
-    ncol(df) == 0 && return Bool[]
+    nrow(df) == 0 && return Bool[]
     res = fill(true, nrow(df))
     cols = ntuple(i -> df[!, i], ncol(df))
     if keep == :first

src/groupeddataframe/utils.jl

@@ -337,7 +337,11 @@ function row_group_slots!(cols::NTuple{N, AbstractVector},
        nt = max(1, lg ÷ 100_000)
     end
     # if there are few rows per group limit the number of threads used
-    nt = clamp(round(Int, (lg / 4) / ngroups - 2), 1, nt)
+    if ngroups == 0
+        nt = 1
+    else
+        nt = clamp(round(Int, (lg / 4) / ngroups - 2), 1, nt)
+    end
 
     seen = fill(false, ngroups)
     seen_vec = Vector{Vector{Bool}}(undef, nt)

test/duplicates.jl

@@ -1,6 +1,6 @@
 module TestDuplicates
 
-using Test, DataFrames, CategoricalArrays, Random
+using Test, DataFrames, CategoricalArrays, Random, PooledArrays
 const ≅ = isequal
 
 @testset "nonunique" begin
@@ -30,15 +30,18 @@ const ≅ = isequal
     @test_throws ArgumentError unique!(df)
     @test_throws ArgumentError unique(df, true)
 
-     pdf = view(DataFrame(a=CategoricalArray(["a", "a", missing, missing, "b", missing, "a", missing]),
-                          b=CategoricalArray(["a", "b", missing, missing, "b", "a", "a", "a"])), :,  :)
+    pdf = view(DataFrame(a=CategoricalArray(["a", "a", missing, missing, "b", missing, "a", missing]),
+                         b=CategoricalArray(["a", "b", missing, missing, "b", "a", "a", "a"])), :,  :)
     updf = DataFrame(a=CategoricalArray(["a", "a", missing, "b", missing]),
                      b=CategoricalArray(["a", "b", missing, "b", "a"]))
     @test nonunique(pdf) == [false, false, false, true, false, false, true, true]
     @test nonunique(updf) == falses(5)
     @test updf ≅ unique(pdf)
     @test_throws ArgumentError unique!(pdf)
     @test_throws ArgumentError unique(pdf, true)
+
+    @test isempty(nonunique(DataFrame(a=PooledArray(Int[]))))
+    @test typeof(nonunique(DataFrame(a=PooledArray(Int[])))) === Vector{Bool}
 end
 
 @testset "nonunique, nonunique, unique! with extra argument" begin

test/grouping.jl

@@ -4531,4 +4531,9 @@ end
     end
 end
 
+@testset "no levels in pooled grouping bug #3393" begin
+    @test isempty(groupby_checked(DataFrame(x=PooledArray([missing])), :x, skipmissing=true))
+    @test isempty(groupby_checked(DataFrame(x=categorical([missing])), :x, skipmissing=true))
+end
+
 end # module