made changes based on suggestions from patrick

Author: Neal Muppidi

pandas/tests/indexing/test_at.py

@@ -99,17 +99,13 @@ def test_at_setitem_categorical_missing(self):
 
         tm.assert_frame_equal(df, expected)
 
-    @pytest.mark.parametrize("row", (to_datetime("2019-01-01"), "2019-01-01"))
+    @pytest.mark.parametrize("row", (Timestamp("2019-01-01"), to_datetime("2019-01-01"), "2019-01-01"))
     def test_at_datetime_index(self, row):
-        df = DataFrame(
-            data=[[1] * 2] * 2,
-            columns=["one", "two"],
-            index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]),
-        )
+        df = DataFrame(data=[[1] * 2], index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]))
+        expected = DataFrame(data=[[0.5, 1], [1.0, 1]], index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]))
 
-        df.at[row, "one"] = 0.5
-        assert df.at[row, "one"] == 0.5
-        assert df["one"].dtype == "float64"
+        df.at[row, 0] = 0.5
+        tm.assert_frame_equal(df, expected)
 
     def test_at_setitem_multiindex(self):
         df = DataFrame(