Backport PR #32166 on branch 1.0.x

doc/source/whatsnew/v1.0.2.rst

@@ -23,6 +23,7 @@ Fixed regressions
 - Fixed regression where :func:`read_pickle` raised a ``UnicodeDecodeError`` when reading a py27 pickle with :class:`MultiIndex` column (:issue:`31988`).
 - Fixed regression in :class:`DataFrame` arithmetic operations with mis-matched columns (:issue:`31623`)
 - Fixed regression in :meth:`GroupBy.agg` calling a user-provided function an extra time on an empty input (:issue:`31760`)
+- Joining on :class:`DatetimeIndex` or :class:`TimedeltaIndex` will preserve ``freq`` in simple cases (:issue:`32166`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/indexes/datetimelike.py

@@ -857,21 +857,16 @@ def _is_convertible_to_index_for_join(cls, other: Index) -> bool:
             return True
         return False
 
-    def _wrap_joined_index(self, joined, other):
+    def _wrap_joined_index(self, joined: np.ndarray, other):
+        assert other.dtype == self.dtype, (other.dtype, self.dtype)
         name = get_op_result_name(self, other)
-        if (
-            isinstance(other, type(self))
-            and self.freq == other.freq
-            and self._can_fast_union(other)
-        ):
-            joined = self._shallow_copy(joined)
-            joined.name = name
-            return joined
-        else:
-            kwargs = {}
-            if hasattr(self, "tz"):
-                kwargs["tz"] = getattr(other, "tz", None)
-            return self._simple_new(joined, name, **kwargs)
+
+        freq = self.freq if self._can_fast_union(other) else None
+        new_data = type(self._data)._simple_new(  # type: ignore
+            joined, dtype=self.dtype, freq=freq
+        )
+
+        return type(self)._simple_new(new_data, name=name)
 
 
 class DatetimelikeDelegateMixin(PandasDelegate):

pandas/tests/indexes/datetimes/test_join.py

@@ -0,0 +1,144 @@
+from datetime import datetime
+
+import numpy as np
+import pytest
+
+from pandas import DatetimeIndex, Index, Timestamp, date_range, to_datetime
+import pandas._testing as tm
+
+from pandas.tseries.offsets import BDay, BMonthEnd
+
+
+class TestJoin:
+    def test_does_not_convert_mixed_integer(self):
+        df = tm.makeCustomDataframe(
+            10,
+            10,
+            data_gen_f=lambda *args, **kwargs: np.random.randn(),
+            r_idx_type="i",
+            c_idx_type="dt",
+        )
+        cols = df.columns.join(df.index, how="outer")
+        joined = cols.join(df.columns)
+        assert cols.dtype == np.dtype("O")
+        assert cols.dtype == joined.dtype
+        tm.assert_numpy_array_equal(cols.values, joined.values)
+
+    def test_join_self(self, join_type):
+        index = date_range("1/1/2000", periods=10)
+        joined = index.join(index, how=join_type)
+        assert index is joined
+
+    def test_join_with_period_index(self, join_type):
+        df = tm.makeCustomDataframe(
+            10,
+            10,
+            data_gen_f=lambda *args: np.random.randint(2),
+            c_idx_type="p",
+            r_idx_type="dt",
+        )
+        s = df.iloc[:5, 0]
+
+        expected = df.columns.astype("O").join(s.index, how=join_type)
+        result = df.columns.join(s.index, how=join_type)
+        tm.assert_index_equal(expected, result)
+
+    def test_join_object_index(self):
+        rng = date_range("1/1/2000", periods=10)
+        idx = Index(["a", "b", "c", "d"])
+
+        result = rng.join(idx, how="outer")
+        assert isinstance(result[0], Timestamp)
+
+    def test_join_utc_convert(self, join_type):
+        rng = date_range("1/1/2011", periods=100, freq="H", tz="utc")
+
+        left = rng.tz_convert("US/Eastern")
+        right = rng.tz_convert("Europe/Berlin")
+
+        result = left.join(left[:-5], how=join_type)
+        assert isinstance(result, DatetimeIndex)
+        assert result.tz == left.tz
+
+        result = left.join(right[:-5], how=join_type)
+        assert isinstance(result, DatetimeIndex)
+        assert result.tz.zone == "UTC"
+
+    @pytest.mark.parametrize("sort", [None, False])
+    def test_datetimeindex_union_join_empty(self, sort):
+        dti = date_range(start="1/1/2001", end="2/1/2001", freq="D")
+        empty = Index([])
+
+        result = dti.union(empty, sort=sort)
+        expected = dti.astype("O")
+        tm.assert_index_equal(result, expected)
+
+        result = dti.join(empty)
+        assert isinstance(result, DatetimeIndex)
+        tm.assert_index_equal(result, dti)
+
+    def test_join_nonunique(self):
+        idx1 = to_datetime(["2012-11-06 16:00:11.477563", "2012-11-06 16:00:11.477563"])
+        idx2 = to_datetime(["2012-11-06 15:11:09.006507", "2012-11-06 15:11:09.006507"])
+        rs = idx1.join(idx2, how="outer")
+        assert rs.is_monotonic
+
+    @pytest.mark.parametrize("freq", ["B", "C"])
+    def test_outer_join(self, freq):
+        # should just behave as union
+        start, end = datetime(2009, 1, 1), datetime(2010, 1, 1)
+        rng = date_range(start=start, end=end, freq=freq)
+
+        # overlapping
+        left = rng[:10]
+        right = rng[5:10]
+
+        the_join = left.join(right, how="outer")
+        assert isinstance(the_join, DatetimeIndex)
+
+        # non-overlapping, gap in middle
+        left = rng[:5]
+        right = rng[10:]
+
+        the_join = left.join(right, how="outer")
+        assert isinstance(the_join, DatetimeIndex)
+        assert the_join.freq is None
+
+        # non-overlapping, no gap
+        left = rng[:5]
+        right = rng[5:10]
+
+        the_join = left.join(right, how="outer")
+        assert isinstance(the_join, DatetimeIndex)
+
+        # overlapping, but different offset
+        other = date_range(start, end, freq=BMonthEnd())
+
+        the_join = rng.join(other, how="outer")
+        assert isinstance(the_join, DatetimeIndex)
+        assert the_join.freq is None
+
+    def test_naive_aware_conflicts(self):
+        start, end = datetime(2009, 1, 1), datetime(2010, 1, 1)
+        naive = date_range(start, end, freq=BDay(), tz=None)
+        aware = date_range(start, end, freq=BDay(), tz="Asia/Hong_Kong")
+
+        msg = "tz-naive.*tz-aware"
+        with pytest.raises(TypeError, match=msg):
+            naive.join(aware)
+
+        with pytest.raises(TypeError, match=msg):
+            aware.join(naive)
+
+    @pytest.mark.parametrize("tz", [None, "US/Pacific"])
+    def test_join_preserves_freq(self, tz):
+        # GH#32157
+        dti = date_range("2016-01-01", periods=10, tz=tz)
+        result = dti[:5].join(dti[5:], how="outer")
+        assert result.freq == dti.freq
+        tm.assert_index_equal(result, dti)
+
+        result = dti[:5].join(dti[6:], how="outer")
+        assert result.freq is None
+        expected = dti.delete(5)
+        tm.assert_index_equal(result, expected)

pandas/tests/indexes/timedeltas/test_join.py

@@ -0,0 +1,49 @@
+import numpy as np
+
+from pandas import Index, Timedelta, timedelta_range
+import pandas._testing as tm
+
+
+class TestJoin:
+    def test_append_join_nondatetimeindex(self):
+        rng = timedelta_range("1 days", periods=10)
+        idx = Index(["a", "b", "c", "d"])
+
+        result = rng.append(idx)
+        assert isinstance(result[0], Timedelta)
+
+        # it works
+        rng.join(idx, how="outer")
+
+    def test_join_self(self, join_type):
+        index = timedelta_range("1 day", periods=10)
+        joined = index.join(index, how=join_type)
+        tm.assert_index_equal(index, joined)
+
+    def test_does_not_convert_mixed_integer(self):
+        df = tm.makeCustomDataframe(
+            10,
+            10,
+            data_gen_f=lambda *args, **kwargs: np.random.randn(),
+            r_idx_type="i",
+            c_idx_type="td",
+        )
+        str(df)
+
+        cols = df.columns.join(df.index, how="outer")
+        joined = cols.join(df.columns)
+        assert cols.dtype == np.dtype("O")
+        assert cols.dtype == joined.dtype
+        tm.assert_index_equal(cols, joined)
+
+    def test_join_preserves_freq(self):
+        # GH#32157
+        tdi = timedelta_range("1 day", periods=10)
+        result = tdi[:5].join(tdi[5:], how="outer")
+        assert result.freq == tdi.freq
+        tm.assert_index_equal(result, tdi)
+
+        result = tdi[:5].join(tdi[6:], how="outer")
+        assert result.freq is None
+        expected = tdi.delete(5)
+        tm.assert_index_equal(result, expected)