From 91a7ebb0b7dc3aa3c077c64ebc30d07a87c64cb8 Mon Sep 17 00:00:00 2001
From: Richard Shadrach <45562402+rhshadrach@users.noreply.github.com>
Date: Mon, 28 Jun 2021 10:27:50 -0400
Subject: [PATCH] Backport PR #42254: REGR: groupby with as_index=False on an
 empty frame

---
 pandas/core/groupby/generic.py       |  4 +++-
 pandas/tests/groupby/conftest.py     |  5 +++++
 pandas/tests/groupby/test_groupby.py | 23 +++++++++++++----------
 3 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index 5cb0eac5d9074..18c84d9aa88bf 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -1676,7 +1676,9 @@ def _wrap_transformed_output(
 
     def _wrap_agged_manager(self, mgr: Manager2D) -> DataFrame:
         if not self.as_index:
-            index = Index(range(mgr.shape[1]))
+            # GH 41998 - empty mgr always gets index of length 0
+            rows = mgr.shape[1] if mgr.shape[0] > 0 else 0
+            index = Index(range(rows))
             mgr.set_axis(1, index)
             result = self.obj._constructor(mgr)
 
diff --git a/pandas/tests/groupby/conftest.py b/pandas/tests/groupby/conftest.py
index 9e7d6d2a7c5bd..d699d05963b46 100644
--- a/pandas/tests/groupby/conftest.py
+++ b/pandas/tests/groupby/conftest.py
@@ -12,6 +12,11 @@
 )
 
 
+@pytest.fixture(params=[True, False])
+def as_index(request):
+    return request.param
+
+
 @pytest.fixture
 def mframe():
     index = MultiIndex(
diff --git a/pandas/tests/groupby/test_groupby.py b/pandas/tests/groupby/test_groupby.py
index 89944e2a745e4..0181481b29c44 100644
--- a/pandas/tests/groupby/test_groupby.py
+++ b/pandas/tests/groupby/test_groupby.py
@@ -15,6 +15,7 @@
     Grouper,
     Index,
     MultiIndex,
+    RangeIndex,
     Series,
     Timestamp,
     date_range,
@@ -2360,19 +2361,21 @@ def test_groupby_all_nan_groups_drop():
     tm.assert_series_equal(result, expected)
 
 
-def test_groupby_empty_multi_column():
-    # GH 15106
+@pytest.mark.parametrize("numeric_only", [True, False])
+def test_groupby_empty_multi_column(as_index, numeric_only):
+    # GH 15106 & GH 41998
     df = DataFrame(data=[], columns=["A", "B", "C"])
-    gb = df.groupby(["A", "B"])
-    result = gb.sum(numeric_only=False)
-    expected = DataFrame(
-        [], columns=["C"], index=MultiIndex([[], []], [[], []], names=["A", "B"])
-    )
+    gb = df.groupby(["A", "B"], as_index=as_index)
+    result = gb.sum(numeric_only=numeric_only)
+    if as_index:
+        index = MultiIndex([[], []], [[], []], names=["A", "B"])
+        columns = ["C"] if not numeric_only else []
+    else:
+        index = RangeIndex(0)
+        columns = ["A", "B", "C"] if not numeric_only else ["A", "B"]
+    expected = DataFrame([], columns=columns, index=index)
     tm.assert_frame_equal(result, expected)
 
-    result = gb.sum(numeric_only=True)
-    tm.assert_frame_equal(result, expected[[]])
-
 
 def test_groupby_filtered_df_std():
     # GH 16174