TST: regression test for groupby with datetime and timedelta (#15562) (#29063)

Author: jhereth

pandas/tests/groupby/test_apply.py

@@ -543,6 +543,33 @@ def predictions(tool):
     tm.assert_series_equal(expected, result)
 
 
+def test_apply_aggregating_timedelta_and_datetime():
+    # Regression test for GH 15562
+    # The following groupby caused ValueErrors and IndexErrors pre 0.20.0
+
+    df = pd.DataFrame(
+        {
+            "clientid": ["A", "B", "C"],
+            "datetime": [np.datetime64("2017-02-01 00:00:00")] * 3,
+        }
+    )
+    df["time_delta_zero"] = df.datetime - df.datetime
+    result = df.groupby("clientid").apply(
+        lambda ddf: pd.Series(
+            dict(clientid_age=ddf.time_delta_zero.min(), date=ddf.datetime.min())
+        )
+    )
+    expected = pd.DataFrame(
+        {
+            "clientid": ["A", "B", "C"],
+            "clientid_age": [np.timedelta64(0, "D")] * 3,
+            "date": [np.datetime64("2017-02-01 00:00:00")] * 3,
+        }
+    ).set_index("clientid")
+
+    tm.assert_frame_equal(result, expected)
+
+
 def test_time_field_bug():
     # Test a fix for the following error related to GH issue 11324 When
     # non-key fields in a group-by dataframe contained time-based fields