restore + change test_evaluate_with_empty_groups

Author: arw2019

pandas/tests/groupby/test_grouping.py

@@ -610,6 +610,33 @@ def test_list_grouper_with_nat(self):
         expected = {pd.Timestamp("2011-01-01"): 365}
         tm.assert_dict_equal(result.groups, expected)
 
+    @pytest.mark.parametrize(
+        "func,expected",
+        [
+            (
+                "transform",
+                pd.Series(name=2, dtype=np.float64, index=pd.Index([], dtype="object")),
+            ),
+            (
+                "agg",
+                pd.Series(name=2, dtype=np.float64, index=pd.Float64Index([], name=1)),
+            ),
+            (
+                "apply",
+                pd.Series(name=2, dtype=np.float64, index=pd.Float64Index([], name=1)),
+            ),
+        ],
+    )
+    def test_evaluate_with_empty_groups(self, func, expected):
+        # 26208
+        # test transform'ing empty groups
+        # (not testing other agg fns, because they return
+        # different index objects.
+        df = pd.DataFrame({1: [], 2: []})
+        g = df.groupby(1)
+        result = getattr(g[2], func)(lambda x: x)
+        tm.assert_series_equal(result, expected)
+
     def test_groupby_empty(self):
         # https://github.com/pandas-dev/pandas/issues/27190
         s = pd.Series([], name="name", dtype="float64")