From 8ea2edfa4a5d0d75470b705bd1d2f4a6f74eda3c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bogumi=C5=82=20Kami=C5=84ski?= <bkamins@sgh.waw.pl>
Date: Thu, 24 Sep 2020 10:20:09 +0200
Subject: [PATCH] add only (#2449)

---
 docs/src/lib/functions.md                  |  1 +
 src/DataFrames.jl                          |  7 +++++++
 src/abstractdataframe/abstractdataframe.jl | 10 ++++++++++
 test/dataframe.jl                          |  7 ++++++-
 4 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/docs/src/lib/functions.md b/docs/src/lib/functions.md
index 62f57681c1..a3e154d2c1 100644
--- a/docs/src/lib/functions.md
+++ b/docs/src/lib/functions.md
@@ -99,6 +99,7 @@ filter
 filter!
 first
 last
+only
 nonunique
 unique
 unique!
diff --git a/src/DataFrames.jl b/src/DataFrames.jl
index 35653e2129..6a8b7db008 100644
--- a/src/DataFrames.jl
+++ b/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")
 
diff --git a/src/abstractdataframe/abstractdataframe.jl b/src/abstractdataframe/abstractdataframe.jl
index ba7a68d76a..46aee96257 100644
--- a/src/abstractdataframe/abstractdataframe.jl
+++ b/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)
 
diff --git a/test/dataframe.jl b/test/dataframe.jl
index 43bf72b450..dddc6ef28d 100644
--- a/test/dataframe.jl
+++ b/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