Implement skipmissing argument to levels

src/array.jl

@@ -753,14 +753,29 @@ end
 leveltype(::Type{T}) where {T <: CategoricalArray} = leveltype(nonmissingtype(eltype(T)))
 
 """
-    levels(x::CategoricalArray)
+    levels(x::CategoricalArray; skipmissing=true)
     levels(x::CategoricalValue)
 
 Return the levels of categorical array or value `x`.
 This may include levels which do not actually appear in the data
 (see [`droplevels!`](@ref)).
+`missing` will be included only if it appears in the data and
+`skipmissing=false` is passed.
+
+The returned vector is an internal field of `x` which must not be mutated
+as doing so would corrupt it.
 """
-DataAPI.levels(A::CategoricalArray) = levels(A.pool)
+@inline function DataAPI.levels(A::CatArrOrSub{T}; skipmissing::Bool=true) where T
+    if eltype(A) >: Missing && !skipmissing
+        if any(==(0), refs(A))
+            T[levels(pool(A)); missing]
+        else
+            convert(Vector{T}, levels(pool(A)))
+        end
+    else
+        levels(pool(A))
+    end
+end
 
 """
     levels!(A::CategoricalArray, newlevels::Vector; allowmissing::Bool=false)

src/subarray.jl

@@ -1,6 +1,5 @@
 # delegate methods for SubArrays to support view
 
-DataAPI.levels(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray} = levels(parent(sa))
 isordered(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray} = isordered(parent(sa))
 # This method cannot support allowmissing=true since that would modify the parent
 levels!(sa::SubArray{T,N,P}, newlevels::Vector) where {T,N,P<:CategoricalArray} =

test/13_arraycommon.jl

@@ -2260,4 +2260,32 @@ end
         Vector{CategoricalVector{<:Any, <:Integer, <:Any, <:Any, Union{}}}
 end
 
+@testset "levels with skipmissing argument" begin
+    for x in (categorical(["a", "b", "a"], levels=["b", "c", "a"]),
+              view(categorical(["c", "b", "a"], levels=["b", "c", "a"]), 2:3))
+        @test @inferred(levels(x)) == ["b", "c", "a"]
+        @test @inferred(levels(x, skipmissing=true)) == ["b", "c", "a"]
+        @test @inferred(levels(x, skipmissing=false)) == ["b", "c", "a"]
+    end
+
+    for x in (categorical(Union{String, Missing}["a", "b", "a"], levels=["b", "c", "a"]),
+              view(categorical(Union{String, Missing}["c", "b", "a"], levels=["b", "c", "a"]), 2:3),
+              view(categorical(Union{String, Missing}[missing, "b", "a"], levels=["b", "c", "a"]), 2:3))
+        @test @inferred(levels(x)) == ["b", "c", "a"]
+        @test levels(x, skipmissing=true) == ["b", "c", "a"]
+        @test levels(x, skipmissing=true) isa Vector{String}
+        @test levels(x, skipmissing=false) == ["b", "c", "a"]
+        @test levels(x, skipmissing=false) isa Vector{Union{String, Missing}}
+    end
+
+    for x in (categorical(Union{String, Missing}["a", "b", missing], levels=["b", "c", "a"]),
+              view(categorical(Union{String, Missing}["c", "b", missing], levels=["b", "c", "a"]), 2:3))
+        @test @inferred(levels(x)) == ["b", "c", "a"]
+        @test levels(x, skipmissing=true) == ["b", "c", "a"]
+        @test levels(x, skipmissing=true) isa Vector{String}
+        @test levels(x, skipmissing=false) ≅ ["b", "c", "a", missing]
+        @test levels(x, skipmissing=false) isa Vector{Union{String, Missing}}
+    end
+end
+
 end