fix #39556 (infer_freq not working with freq="H" and DST (#39644)

* fix #39556:
- check that the delta are unique before checking if the are day multiples
- add test with freq="H" that raises the bug

* fix lint

* when freq=="B", the deltas are not unique (1 or 3 days) => change by taking for delta the minimum of deltas and checking delta is not null

* add whatsnew entry

* as self.deltas is already ordered, deltas[0] is the minimum delta

Co-authored-by: GFJ138 <[email protected]>

Author: sdementen

doc/source/whatsnew/v1.3.0.rst

@@ -300,6 +300,7 @@ Datetimelike
 - Bug in :meth:`Timestamp.round`, :meth:`Timestamp.floor`, :meth:`Timestamp.ceil` for values near the implementation bounds of :class:`Timestamp` (:issue:`39244`)
 - Bug in :meth:`Timedelta.round`, :meth:`Timedelta.floor`, :meth:`Timedelta.ceil` for values near the implementation bounds of :class:`Timedelta` (:issue:`38964`)
 - Bug in :func:`date_range` incorrectly creating :class:`DatetimeIndex` containing ``NaT`` instead of raising ``OutOfBoundsDatetime`` in corner cases (:issue:`24124`)
+- Bug in :func:`infer_freq` incorrectly fails to infer 'H' frequency of :class:`DatetimeIndex` if the latter has a timezone and crosses DST boundaries (:issue:`39556`)
 
 Timedelta
 ^^^^^^^^^

pandas/tests/tseries/frequencies/test_inference.py

@@ -280,7 +280,7 @@ def test_infer_freq_tz(tz_naive_fixture, expected, dates):
     ],
 )
 @pytest.mark.parametrize(
-    "freq", ["3H", "10T", "3601S", "3600001L", "3600000001U", "3600000000001N"]
+    "freq", ["H", "3H", "10T", "3601S", "3600001L", "3600000001U", "3600000000001N"]
 )
 def test_infer_freq_tz_transition(tz_naive_fixture, date_pair, freq):
     # see gh-8772

pandas/tseries/frequencies.py

@@ -246,16 +246,17 @@ def get_freq(self) -> Optional[str]:
             return None
 
         delta = self.deltas[0]
-        if _is_multiple(delta, _ONE_DAY):
+        if delta and _is_multiple(delta, _ONE_DAY):
             return self._infer_daily_rule()
 
         # Business hourly, maybe. 17: one day / 65: one weekend
         if self.hour_deltas in ([1, 17], [1, 65], [1, 17, 65]):
             return "BH"
+
         # Possibly intraday frequency.  Here we use the
         # original .asi8 values as the modified values
         # will not work around DST transitions.  See #8772
-        elif not self.is_unique_asi8:
+        if not self.is_unique_asi8:
             return None
 
         delta = self.deltas_asi8[0]