TST : add test for groupby aggregation dtype (#43915)

Author: OlivierCavadenti

pandas/tests/groupby/aggregate/test_aggregate.py

@@ -1327,6 +1327,28 @@ def func(ser):
     tm.assert_frame_equal(res, expected)
 
 
+def test_groupby_aggregate_directory(reduction_func):
+    # GH#32793
+    if reduction_func in ["corrwith", "nth"]:
+        return None
+
+    obj = DataFrame([[0, 1], [0, np.nan]])
+
+    result_reduced_series = obj.groupby(0).agg(reduction_func)
+    result_reduced_frame = obj.groupby(0).agg({1: reduction_func})
+
+    if reduction_func in ["size", "ngroup"]:
+        # names are different: None / 1
+        tm.assert_series_equal(
+            result_reduced_series, result_reduced_frame[1], check_names=False
+        )
+    else:
+        tm.assert_frame_equal(result_reduced_series, result_reduced_frame)
+        tm.assert_series_equal(
+            result_reduced_series.dtypes, result_reduced_frame.dtypes
+        )
+
+
 def test_group_mean_timedelta_nat():
     # GH43132
     data = Series(["1 day", "3 days", "NaT"], dtype="timedelta64[ns]")