TST: Additional to iloc setitem

This tests make sure when converting multiple columns to datetimes
and when assiging back it remains as datetime not as unix date
as mentioned in GH pandas-dev#20511.

Author: parthi-siva

pandas/tests/indexing/test_iloc.py

@@ -26,6 +26,7 @@
     date_range,
     interval_range,
     isna,
+    to_datetime,
 )
 import pandas._testing as tm
 from pandas.api.types import is_scalar
@@ -1188,6 +1189,17 @@ def test_iloc_getitem_int_single_ea_block_view(self):
         arr[2] = arr[-1]
         assert ser[0] == arr[-1]
 
+    def test_iloc_setitem_multicolumn_to_datetime(self):
+
+        # GH#20511
+        df = DataFrame({"A": ["2022-01-01", "2022-01-02"], "B": ["2021", "2022"]})
+
+        df.iloc[:, [0]] = DataFrame({"A": to_datetime(["2021", "2022"])})
+        expected = DataFrame(
+            {"A": to_datetime(["2021", "2022"]), "B": ["2021", "2022"]}
+        )
+        tm.assert_frame_equal(df, expected, check_dtype=False)
+
 
 class TestILocErrors:
     # NB: this test should work for _any_ Series we can pass as
@@ -1363,6 +1375,17 @@ def test_frame_iloc_setitem_callable(self):
         exp.iloc[[1, 3], [0]] = [-5, -5]
         tm.assert_frame_equal(res, exp)
 
+    def test_frame_iloc_setitem_callable_multicolumn_to_datetime(self):
+
+        # GH#20511
+        df = DataFrame({"A": ["2022-01-01", "2022-01-02"], "B": ["2021", "2022"]})
+
+        df.iloc[:, [0]] = df.iloc[:, [0]].apply(to_datetime)
+        expected = DataFrame(
+            {"A": to_datetime(["2022-01-01", "2022-01-02"]), "B": ["2021", "2022"]}
+        )
+        tm.assert_frame_equal(df, expected, check_dtype=False)
+
 
 class TestILocSeries:
     def test_iloc(self):