BUG: inferred_freq with non-nano (#55609)

* BUG: inferred_freq with non-nano

* GH ref

Author: jbrockmendel

doc/source/whatsnew/v2.2.0.rst

@@ -410,6 +410,7 @@ Styler
 Other
 ^^^^^
 - Bug in :func:`cut` incorrectly allowing cutting of timezone-aware datetimes with timezone-naive bins (:issue:`54964`)
+- Bug in :func:`infer_freq` and :meth:`DatetimeIndex.inferred_freq` with weekly frequencies and non-nanosecond resolutions (:issue:`55609`)
 - Bug in :meth:`DataFrame.apply` where passing ``raw=True`` ignored ``args`` passed to the applied function (:issue:`55009`)
 - Bug in rendering ``inf`` values inside a a :class:`DataFrame` with the ``use_inf_as_na`` option enabled (:issue:`55483`)
 - Bug in rendering a :class:`Series` with a :class:`MultiIndex` when one of the index level's names is 0 not having that name displayed (:issue:`55415`)

pandas/tests/tseries/frequencies/test_inference.py

@@ -229,10 +229,11 @@ def test_infer_freq_index(freq, expected):
         }.items()
     ),
 )
-def test_infer_freq_tz(tz_naive_fixture, expected, dates):
-    # see gh-7310
+@pytest.mark.parametrize("unit", ["s", "ms", "us", "ns"])
+def test_infer_freq_tz(tz_naive_fixture, expected, dates, unit):
+    # see gh-7310, GH#55609
     tz = tz_naive_fixture
-    idx = DatetimeIndex(dates, tz=tz)
+    idx = DatetimeIndex(dates, tz=tz).as_unit(unit)
     assert idx.inferred_freq == expected
 
 

pandas/tseries/frequencies.py

@@ -276,7 +276,7 @@ def fields(self) -> np.ndarray:  # structured array of fields
 
     @cache_readonly
     def rep_stamp(self) -> Timestamp:
-        return Timestamp(self.i8values[0])
+        return Timestamp(self.i8values[0], unit=self.index.unit)
 
     def month_position_check(self) -> str | None:
         return month_position_check(self.fields, self.index.dayofweek)