TST: Add regression tests for concat with non-ns DatetimeIndex units (GH#58471)

Author: skalwaghe-56

pandas/tests/reshape/concat/test_datetimes.py

@@ -274,6 +274,51 @@ def test_concat_NaT_series_dataframe_all_NaT(self, tz1, tz2):
         result = concat([first, second])
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize("unit", ["ns", "us", "ms", "s"])
+    def test_concat_series_columns_nonoverlap_5min_units(self, unit):
+        # GH#58471
+        idx1 = date_range("2024-01-01", periods=24 * 12, freq="5min", unit=unit)
+        idx2 = date_range("2024-01-02", periods=24 * 12, freq="5min", unit=unit)
+        s1 = Series(np.arange(len(idx1)), index=idx1, name="a")
+        s2 = Series(np.arange(len(idx2)), index=idx2, name="b")
+        result = concat([s1, s2], axis=1)
+        expected_index = date_range(
+            "2024-01-01", "2024-01-02 23:55", freq="5min", unit=unit
+        )
+        expected_data = np.full((len(expected_index), 2), np.nan)
+        expected_data[: len(idx1), 0] = np.arange(len(idx1))
+        expected_data[len(idx1) :, 1] = np.arange(len(idx2))
+        expected = DataFrame(expected_data, index=expected_index, columns=["a", "b"])
+        tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("unit", ["ns", "us", "ms", "s"])
+    def test_concat_series_columns_month_end_units_order_insensitive(self, unit):
+        # GH#58471
+        idx1 = date_range(start="2015-01-31", end="2016-01-31", freq="ME", unit=unit)
+        idx2 = date_range(start="2015-02-28", end="2016-02-29", freq="ME", unit=unit)
+        s1 = Series(np.arange(len(idx1)), index=idx1, name="m1")
+        s2 = Series(np.arange(len(idx2)), index=idx2, name="m2")
+        result1 = concat([s1, s2], axis=1)
+        result2 = concat([s2, s1], axis=1)
+
+        expected_index = idx1.union(idx2)
+        expected_data = np.full((len(expected_index), 2), np.nan)
+        for i, date in enumerate(expected_index):
+            if date in idx1:
+                expected_data[i, 0] = s1.loc[date]
+            if date in idx2:
+                expected_data[i, 1] = s2.loc[date]
+
+        expected1 = DataFrame(expected_data, index=expected_index, columns=["m1", "m2"])
+        tm.assert_frame_equal(result1, expected1)
+
+        expected2 = DataFrame(
+            expected_data[:, [1, 0]],
+            index=expected_index,
+            columns=["m2", "m1"],
+        )
+        tm.assert_frame_equal(result2, expected2)
+
 
 class TestTimezoneConcat:
     def test_concat_tz_series(self):