From 5fe02971f8bc40a6bbb742f1f376b3650c3c8eaa Mon Sep 17 00:00:00 2001
From: Calvin Ho <18680207+calvh@users.noreply.github.com>
Date: Thu, 26 Aug 2021 23:20:52 -0400
Subject: [PATCH] TST: Add test for col names during groupby().agg()

Column names should consistently be retained when using df.groupby().agg()
---
 .../tests/groupby/aggregate/test_aggregate.py | 32 +++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/pandas/tests/groupby/aggregate/test_aggregate.py b/pandas/tests/groupby/aggregate/test_aggregate.py
index 0a693967fbb19..51de097a1d30f 100644
--- a/pandas/tests/groupby/aggregate/test_aggregate.py
+++ b/pandas/tests/groupby/aggregate/test_aggregate.py
@@ -1274,3 +1274,35 @@ def func(ser):
 
     expected = DataFrame([[1.0]], index=[1])
     tm.assert_frame_equal(res, expected)
+
+
+def test_groupby_agg_column_names():
+    # GH42332
+
+    df = DataFrame(columns=["id1", "id2", "time", "values"], dtype="int").groupby(
+        ["id1", "id2"]
+    )
+
+    df_sum_idx = df.sum().index.names
+
+    df_agg1_idx = df.agg(
+        **{"start": pd.NamedAgg(column="time", aggfunc="min")}
+    ).index.names
+
+    df_agg2_idx = df.agg(
+        **{
+            "start": pd.NamedAgg(column="time", aggfunc="min"),
+            "peak_time": pd.NamedAgg(column="values", aggfunc="idxmax"),
+        }
+    ).index.names
+
+    df_agg3_idx = df.agg(
+        **{"peak_time": pd.NamedAgg(column="values", aggfunc="idxmax")}
+    ).index.names
+
+    expected = ["id1", "id2"]
+
+    assert df_sum_idx == expected
+    assert df_agg1_idx == expected
+    assert df_agg2_idx == expected
+    assert df_agg3_idx == expected