BUG: Timestamp(ambiguous, tz=from_pytz) not raising

doc/source/whatsnew/v2.2.0.rst

@@ -339,6 +339,7 @@ Timedelta
 Timezones
 ^^^^^^^^^
 - Bug in :class:`AbstractHolidayCalendar` where timezone data was not propagated when computing holiday observances (:issue:`54580`)
+- Bug in :class:`Timestamp` construction with an ambiguous value and a ``pytz`` timezone failing to raise ``pytz.AmbiguousTimeError`` (:issue:`55657`)
 -
 
 Numeric

pandas/_libs/tslibs/conversion.pyx

@@ -638,7 +638,7 @@ cdef datetime _localize_pydatetime(datetime dt, tzinfo tz):
     """
     try:
         # datetime.replace with pytz may be incorrect result
-        return tz.localize(dt)
+        return tz.localize(dt, is_dst=None)
     except AttributeError:
         return dt.replace(tzinfo=tz)
 

pandas/tests/indexes/datetimes/test_constructors.py

@@ -1013,6 +1013,36 @@ def test_dti_convert_datetime_list(self, tzstr):
         dr2 = DatetimeIndex(list(dr), name="foo", freq="D")
         tm.assert_index_equal(dr, dr2)
 
+    def test_dti_ambiguous_matches_timestamp(self):
+        # GH#47471 check that we get the same raising behavior in the DTI
+        # constructor and Timestamp constructor
+        dtstr = "2013-11-03 01:59:59.999999"
+        dtobj = Timestamp(dtstr).to_pydatetime()
+
+        tz = pytz.timezone("US/Eastern")
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            Timestamp(dtstr, tz=tz)
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            Timestamp(dtobj, tz=tz)
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            DatetimeIndex([dtstr], tz=tz)
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            DatetimeIndex([dtobj], tz=tz)
+
+        tz2 = gettz("US/Eastern")
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            Timestamp(dtstr, tz=tz2)
+        # FIXME: The Timestamp constructor here behaves differently than all
+        #  the other cases bc with dateutil/zoneinfo tzinfos we implicitly
+        #  get fold=0. Having this raise is not important, but having the
+        #  behavior be consistent across cases is.
+        # with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+        #    Timestamp(dtobj, tz=tz2)
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            DatetimeIndex([dtstr], tz=tz2)
+        with pytest.raises(pytz.AmbiguousTimeError, match=dtstr):
+            DatetimeIndex([dtobj], tz=tz2)
+
 
 class TestTimeSeries:
     def test_dti_constructor_preserve_dti_freq(self):

pandas/tests/tseries/offsets/test_dst.py

@@ -29,7 +29,10 @@
     YearBegin,
     YearEnd,
 )
+from pandas.errors import PerformanceWarning
 
+from pandas import DatetimeIndex
+import pandas._testing as tm
 from pandas.util.version import Version
 
 # error: Module has no attribute "__version__"
@@ -83,6 +86,29 @@ def _test_all_offsets(self, n, **kwds):
     def _test_offset(self, offset_name, offset_n, tstart, expected_utc_offset):
         offset = DateOffset(**{offset_name: offset_n})
 
+        if (
+            offset_name in ["hour", "minute", "second", "microsecond"]
+            and offset_n == 1
+            and tstart == Timestamp("2013-11-03 01:59:59.999999-0500", tz="US/Eastern")
+        ):
+            # This addition results in an ambiguous wall time
+            err_msg = {
+                "hour": "2013-11-03 01:59:59.999999",
+                "minute": "2013-11-03 01:01:59.999999",
+                "second": "2013-11-03 01:59:01.999999",
+                "microsecond": "2013-11-03 01:59:59.000001",
+            }[offset_name]
+            with pytest.raises(pytz.AmbiguousTimeError, match=err_msg):
+                tstart + offset
+            # While we're here, let's check that we get the same behavior in a
+            #  vectorized path
+            dti = DatetimeIndex([tstart])
+            warn_msg = "Non-vectorized DateOffset"
+            with pytest.raises(pytz.AmbiguousTimeError, match=err_msg):
+                with tm.assert_produces_warning(PerformanceWarning, match=warn_msg):
+                    dti + offset
+            return
+
         t = tstart + offset
         if expected_utc_offset is not None:
             assert get_utc_offset_hours(t) == expected_utc_offset