Improve allcombinations docstring + minor cleanups after #3256 (#3276)

Author: bkamins

NEWS.md

@@ -32,6 +32,11 @@
   its parent
   ([#3273](https://github.com/JuliaData/DataFrames.jl/pull/3273))
 
+## Performance improvements
+
+* `dropmissing` creates new columns in a single pass if `disallowmissing=true`
+  ([#3256](https://github.com/JuliaData/DataFrames.jl/pull/3256))
+
 # DataFrames.jl v1.4.4 Patch Release Notes
 
 ## Bug fixes

src/abstractdataframe/abstractdataframe.jl

@@ -985,11 +985,19 @@ julia> dropmissing(df, [:x, :y])
 
         # What column indices should disallowmissing be applied to
         cols_inds = BitSet(index(df)[cols])
-        
+
         use_threads = Threads.nthreads() > 1 && ncol(df) > 1 && length(selected_rows) >= 100_000
         @sync for (i, col) in enumerate(eachcol(df))
-            @spawn_or_run use_threads if disallowmissing && (i in cols_inds)
-                new_columns[i] = Missings.disallowmissing(Base.view(col, selected_rows))
+            @spawn_or_run use_threads if disallowmissing && (i in cols_inds) &&
+                          (Missing <: eltype(col) && eltype(col) !== Any)
+                # Perform this path only if column eltype allows missing values
+                # except Any, as nonmissingtype(Any) == Any.
+                # Under these conditions Missings.disallowmissing must allocate
+                # a fresh column
+                col_sel = Base.view(col, selected_rows)
+                new_col = Missings.disallowmissing(col_sel)
+                @assert new_col !== col_sel
+                new_columns[i] = new_col
             else
                 new_columns[i] = col[selected_rows]
             end
@@ -3421,4 +3429,3 @@ function Base.iterate(itr::Iterators.PartitionIterator{<:AbstractDataFrame}, sta
     r = min(state + itr.n - 1, last_idx)
     return view(itr.c, state:r, :), r + 1
 end
-

src/dataframe/dataframe.jl

@@ -1472,6 +1472,7 @@ allcombinations(::Type{DataFrame}, pairs::Pair{<:AbstractString, <:Any}...) =
     allcombinations(DataFrame; kwargs...)
 
 Create a `DataFrame` from all combinations of values in passed arguments.
+The first passed values vary fastest.
 
 Arguments associating a column name with values to expand can be specified
 either as `Pair`s passed as positional arguments, or as keyword arguments.

test/data.jl

@@ -173,12 +173,11 @@ end
     @test isempty(dropmissing!(DataFrame())) && dropmissing!(DataFrame()) isa DataFrame
     df = DataFrame(a=1:3, b=4:6)
     dfv = @view df[:, 2:1]
-    # TODO: re-enable after https://github.com/JuliaData/DataFrames.jl/issues/3272 is resolved
-    # @test isempty(dropmissing(dfv)) && dropmissing(dfv) isa DataFrame
+    @test isempty(dropmissing(dfv)) && dropmissing(dfv) isa DataFrame
     @test_throws ArgumentError dropmissing!(dfv)
-    @test_throws ArgumentError dropmissing(df1, []) 
-    @test_throws ArgumentError dropmissing!(df1, []) 
-    
+    @test_throws ArgumentError dropmissing(df1, [])
+    @test_throws ArgumentError dropmissing!(df1, [])
+
     df = DataFrame(a=[1, missing, 3])
     sdf = view(df, :, :)
     @test dropmissing(sdf) == DataFrame(a=[1, 3])
@@ -248,19 +247,26 @@ end
     # Deterministic drop mask: IF remainder of index position divided by 10 == column index THEN missing
     for i in 1:ncol(df)
         missing_mask = (eachindex(df[!, i]) .% 10) .== i
-        df[missing_mask, i] .= missing 
+        df[missing_mask, i] .= missing
     end
-    
+
     notmissing_rows = [i for i in 1:N_rows if i % 10 == 0 || i % 10 > ncol(df)]
     @test dropmissing(df) ≅ df[notmissing_rows, :]
-    
+
     cols = [:x1, :x2]
     notmissing_rows = [i for i in 1:N_rows if i % 10 == 0 || i % 10 > length(cols)]
     returned = dropmissing(df, cols)
     @test returned ≅ df[notmissing_rows, :]
     @test eltype(returned[:, cols[1]]) == nonmissingtype(eltype(df[:, cols[1]]))
     @test eltype(returned[:, cols[2]]) == nonmissingtype(eltype(df[:, cols[2]]))
     @test eltype(returned[:, ncol(df)]) == eltype(df[:, ncol(df)])
+
+    # correct handling of not propagating views
+    df = DataFrame(a=1:3, b=Any[11, missing, 13])
+    df2 = dropmissing(df)
+    @test df2 == DataFrame(a=[1, 3], b=[11, 13])
+    @test df2.a isa Vector{Int}
+    @test df2.b isa Vector{Any}
 end
 
 @testset "deleteat! https://github.com/JuliaLang/julia/pull/41646 bug workaround" begin