BUG: Fix timezone handling in DatetimeIndex.union (#60080)

Author: brandonmonge

pandas/core/indexes/base.py

@@ -2958,6 +2958,29 @@ def _dti_setop_align_tzs(self, other: Index, setop: str_t) -> tuple[Index, Index
             and self.tz is not None
             and other.tz is not None
         ):
+            if self.tz == other.tz:
+                # GH #60080: Handle union of DatetimeIndex with
+                # the same timezone but different resolutions
+                resolution_order = {
+                    "Y": 1,  # Year
+                    "M": 2,  # Month
+                    "W": 3,  # Week
+                    "D": 4,  # Day
+                    "h": 5,  # Hour
+                    "m": 6,  # Minute
+                    "s": 7,  # Second
+                    "ms": 8,  # Millisecond
+                    "us": 9,  # Microsecond
+                    "ns": 10,  # Nanosecond
+                }
+                # Default to the lowest resolution if unit is unknown
+                self_res = resolution_order.get(self.dtype.unit, 0)
+                other_res = resolution_order.get(other.dtype.unit, 0)
+                # Choose the dtype with higher resolution
+                dtype = self.dtype if self_res >= other_res else other.dtype
+                left = self.astype(dtype, copy=False)
+                right = other.astype(dtype, copy=False)
+                return left, right
             # GH#39328, GH#45357
             left = self.tz_convert("UTC")
             right = other.tz_convert("UTC")

pandas/tests/indexes/datetimes/test_setops.py

@@ -694,3 +694,79 @@ def test_intersection_non_nano_rangelike():
         freq="D",
     )
     tm.assert_index_equal(result, expected)
+
+
+def test_union_preserves_timezone_and_resolution():
+    """
+    GH 60080: Ensure union of DatetimeIndex with the same timezone
+    and differing resolutions results in the higher resolution unit
+    and preserves the timezone.
+    """
+    idx1 = DatetimeIndex(["2020-01-01 10:00:00+05:00"]).astype(
+        "datetime64[us, UTC+05:00]"
+    )
+    idx2 = DatetimeIndex(["2020-01-01 10:00:00+05:00"]).astype(
+        "datetime64[ns, UTC+05:00]"
+    )
+    result = idx1.union(idx2)
+    expected = DatetimeIndex(["2020-01-01 10:00:00+05:00"]).astype(
+        "datetime64[ns, UTC+05:00]"
+    )
+    tm.assert_index_equal(result, expected)
+
+
+def test_union_multiple_entries_same_timezone():
+    """
+    GH 60080: Test union with multiple DatetimeIndex entries having the same timezone
+    and different units, ensuring correct alignment and resolution preservation.
+    """
+    idx1 = DatetimeIndex(
+        ["2023-01-01 10:00:00+05:00", "2023-01-02 10:00:00+05:00"]
+    ).astype("datetime64[us, UTC+05:00]")
+    idx2 = DatetimeIndex(
+        ["2023-01-01 10:00:00+05:00", "2023-01-03 10:00:00+05:00"]
+    ).astype("datetime64[ns, UTC+05:00]")
+    result = idx1.union(idx2)
+    expected = DatetimeIndex(
+        [
+            "2023-01-01 10:00:00+05:00",
+            "2023-01-02 10:00:00+05:00",
+            "2023-01-03 10:00:00+05:00",
+        ]
+    ).astype("datetime64[ns, UTC+05:00]")
+    tm.assert_index_equal(result, expected)
+
+
+def test_union_same_timezone_same_resolution():
+    """
+    GH 60080: Ensure union of DatetimeIndex with the same timezone and
+    resolution is straightforward and retains the resolution.
+    """
+    idx1 = DatetimeIndex(["2022-01-01 15:00:00+05:00"]).astype(
+        "datetime64[ms, UTC+05:00]"
+    )
+    idx2 = DatetimeIndex(["2022-01-01 16:00:00+05:00"]).astype(
+        "datetime64[ms, UTC+05:00]"
+    )
+    result = idx1.union(idx2)
+    expected = DatetimeIndex(
+        ["2022-01-01 15:00:00+05:00", "2022-01-01 16:00:00+05:00"]
+    ).astype("datetime64[ms, UTC+05:00]")
+    tm.assert_index_equal(result, expected)
+
+    def test_union_single_entry():
+        """
+        GH 60080: Ensure union of single-entry DatetimeIndex works as expected
+        with different units and same timezone.
+        """
+        idx1 = DatetimeIndex(["2023-01-01 10:00:00+05:00"]).astype(
+            "datetime64[ms, UTC+05:00]"
+        )
+        idx2 = DatetimeIndex(["2023-01-01 10:00:00+05:00"]).astype(
+            "datetime64[us, UTC+05:00]"
+        )
+        result = idx1.union(idx2)
+        expected = DatetimeIndex(["2023-01-01 10:00:00+05:00"]).astype(
+            "datetime64[us, UTC+05:00]"
+        )
+        tm.assert_index_equal(result, expected)