fix pandas-dev#39556:

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

Author: sebastiendementen

pandas/tests/tseries/frequencies/test_inference.py

@@ -267,7 +267,7 @@ 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

@@ -239,17 +239,17 @@ def get_freq(self) -> Optional[str]:
         if not self.is_monotonic or not self.index._is_unique:
             return None
 
-        delta = self.deltas[0]
-        if _is_multiple(delta, _ONE_DAY):
+        if self.is_unique and _is_multiple(self.deltas[0], _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]
@@ -414,7 +414,7 @@ def _infer_daily_rule(self):
 
 
 def _is_multiple(us, mult: int) -> bool:
-    return us % mult == 0
+    return (us % mult == 0)
 
 
 def _maybe_add_count(base: str, count: float) -> str: