From 8b0ad717d1ec54dd40136817a326b41817ffcb86 Mon Sep 17 00:00:00 2001
From: Dennis Chukwunta <chuksmcdennis@yahoo.com>
Date: Tue, 27 Sep 2022 21:21:24 +0100
Subject: [PATCH] BUG: Fix calling groupBy(...).apply(func) on an empty
 dataframe invokes func  (#48579)

---
 doc/source/whatsnew/v1.5.1.rst     |  1 +
 pandas/core/groupby/ops.py         | 13 ++++++-------
 pandas/tests/groupby/test_apply.py | 25 +++++++++++++++++++++++++
 3 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/doc/source/whatsnew/v1.5.1.rst b/doc/source/whatsnew/v1.5.1.rst
index 852166cbfaa7f..ee66f2f649bc2 100644
--- a/doc/source/whatsnew/v1.5.1.rst
+++ b/doc/source/whatsnew/v1.5.1.rst
@@ -77,6 +77,7 @@ Fixed regressions
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 - Fixed regression causing an ``AttributeError`` during warning emitted if the provided table name in :meth:`DataFrame.to_sql` and the table name actually used in the database do not match (:issue:`48733`)
 - Fixed :meth:`.DataFrameGroupBy.size` not returning a Series when ``axis=1`` (:issue:`48738`)
+- Fixed Regression in :meth:`DataFrameGroupBy.apply` when user defined function is called on an empty dataframe (:issue:`47985`)
 
 .. ---------------------------------------------------------------------------
 
diff --git a/pandas/core/groupby/ops.py b/pandas/core/groupby/ops.py
index 762e135111cf8..04f18369f4fcc 100644
--- a/pandas/core/groupby/ops.py
+++ b/pandas/core/groupby/ops.py
@@ -787,15 +787,14 @@ def apply(
             if not mutated and not _is_indexed_like(res, group_axes, axis):
                 mutated = True
             result_values.append(res)
-
         # getattr pattern for __name__ is needed for functools.partial objects
-        if len(group_keys) == 0 and getattr(f, "__name__", None) not in [
-            "idxmin",
-            "idxmax",
-            "nanargmin",
-            "nanargmax",
+        if len(group_keys) == 0 and getattr(f, "__name__", None) in [
+            "mad",
+            "skew",
+            "sum",
+            "prod",
         ]:
-            # If group_keys is empty, then no function calls have been made,
+            #  If group_keys is empty, then no function calls have been made,
             #  so we will not have raised even if this is an invalid dtype.
             #  So do one dummy call here to raise appropriate TypeError.
             f(data.iloc[:0])
diff --git a/pandas/tests/groupby/test_apply.py b/pandas/tests/groupby/test_apply.py
index b064c12f89c21..47ea6a99ffea9 100644
--- a/pandas/tests/groupby/test_apply.py
+++ b/pandas/tests/groupby/test_apply.py
@@ -1331,3 +1331,28 @@ def test_result_name_when_one_group(name):
     expected = Series([1, 2], name=name)
 
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "method, op",
+    [
+        ("apply", lambda gb: gb.values[-1]),
+        ("apply", lambda gb: gb["b"].iloc[0]),
+        ("agg", "mad"),
+        ("agg", "skew"),
+        ("agg", "prod"),
+        ("agg", "sum"),
+    ],
+)
+def test_empty_df(method, op):
+    # GH 47985
+    empty_df = DataFrame({"a": [], "b": []})
+    gb = empty_df.groupby("a", group_keys=True)
+    group = getattr(gb, "b")
+
+    result = getattr(group, method)(op)
+    expected = Series(
+        [], name="b", dtype="float64", index=Index([], dtype="float64", name="a")
+    )
+
+    tm.assert_series_equal(result, expected)