add only (#2449)

docs/src/lib/functions.md

@@ -99,6 +99,7 @@ filter
 filter!
 first
 last
+only
 nonunique
 unique
 unique!

src/DataFrames.jl

@@ -80,6 +80,13 @@ if VERSION < v"1.2"
     export hasproperty
 end
 
+if isdefined(Base, :only)  # Introduced in 1.4.0
+    import Base.only
+else
+    import Compat.only
+    export only
+end
+
 include("other/utils.jl")
 include("other/index.jl")
 

src/abstractdataframe/abstractdataframe.jl

@@ -434,6 +434,16 @@ end
 ##
 ##############################################################################
 
+"""
+    only(df::AbstractDataFrame)
+
+If `df` has a single row return it as a `DataFrameRow`; otherwise throw `ArgumentError`.
+"""
+function only(df::AbstractDataFrame)
+    nrow(df) != 1 && throw(ArgumentError("data frame must contain exactly 1 row"))
+    return df[1, :]
+end
+
 """
     first(df::AbstractDataFrame)
 

test/dataframe.jl

@@ -1051,7 +1051,7 @@ end
     @inferred ncol(df)
 end
 
-@testset "description" begin
+@testset "first, last and only" begin
     df = DataFrame(A = 1:10)
 
     @test first(df) == df[1, :]
@@ -1063,6 +1063,11 @@ end
     @test first(df, 1) == DataFrame(A = 1)
     @test last(df, 6) == DataFrame(A = 5:10)
     @test last(df, 1) == DataFrame(A = 10)
+
+    @test_throws ArgumentError only(df)
+    @test_throws ArgumentError only(DataFrame())
+    df = DataFrame(a=1, b=2)
+    @test only(df) === df[1, :]
 end
 
 @testset "column conversions" begin