BUG: Fix to_datetime not respecting dayfirst (#58876)

* ENH: Warn when to_datetime falls back to dateutil when dayfirst is passed

* Assert warnings

* Remove warnings and fix functionality

* Add whatsnew, write test

Author: Aloqeely

doc/source/whatsnew/v3.0.0.rst

@@ -509,6 +509,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
 - Bug in :meth:`DatetimeIndex.union` when ``unit`` was non-nanosecond (:issue:`59036`)
 - Bug in :meth:`Series.dt.microsecond` producing incorrect results for pyarrow backed :class:`Series`. (:issue:`59154`)
+- Bug in :meth:`to_datetime` not respecting dayfirst if an uncommon date string was passed. (:issue:`58859`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
 
 Timedelta

pandas/_libs/tslibs/conversion.pyx

@@ -606,37 +606,42 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
         # equiv: datetime.today().replace(tzinfo=tz)
         return convert_datetime_to_tsobject(dt, tz, nanos=0, reso=NPY_FR_us)
     else:
-        string_to_dts_failed = string_to_dts(
-            ts, &dts, &out_bestunit, &out_local,
-            &out_tzoffset, False
-        )
-        if not string_to_dts_failed:
-            reso = get_supported_reso(out_bestunit)
-            check_dts_bounds(&dts, reso)
-            obj = _TSObject()
-            obj.dts = dts
-            obj.creso = reso
-            ival = npy_datetimestruct_to_datetime(reso, &dts)
-
-            if out_local == 1:
-                obj.tzinfo = timezone(timedelta(minutes=out_tzoffset))
-                obj.value = tz_localize_to_utc_single(
-                    ival, obj.tzinfo, ambiguous="raise", nonexistent=None, creso=reso
-                )
-                if tz is None:
-                    check_overflows(obj, reso)
-                    return obj
-                _adjust_tsobject_tz_using_offset(obj, tz)
-                return  obj
-            else:
-                if tz is not None:
-                    # shift for _localize_tso
-                    ival = tz_localize_to_utc_single(
-                        ival, tz, ambiguous="raise", nonexistent=None, creso=reso
+        if not dayfirst:  # GH 58859
+            string_to_dts_failed = string_to_dts(
+                ts, &dts, &out_bestunit, &out_local,
+                &out_tzoffset, False
+            )
+            if not string_to_dts_failed:
+                reso = get_supported_reso(out_bestunit)
+                check_dts_bounds(&dts, reso)
+                obj = _TSObject()
+                obj.dts = dts
+                obj.creso = reso
+                ival = npy_datetimestruct_to_datetime(reso, &dts)
+
+                if out_local == 1:
+                    obj.tzinfo = timezone(timedelta(minutes=out_tzoffset))
+                    obj.value = tz_localize_to_utc_single(
+                        ival,
+                        obj.tzinfo,
+                        ambiguous="raise",
+                        nonexistent=None,
+                        creso=reso,
                     )
-                obj.value = ival
-                maybe_localize_tso(obj, tz, obj.creso)
-                return obj
+                    if tz is None:
+                        check_overflows(obj, reso)
+                        return obj
+                    _adjust_tsobject_tz_using_offset(obj, tz)
+                    return  obj
+                else:
+                    if tz is not None:
+                        # shift for _localize_tso
+                        ival = tz_localize_to_utc_single(
+                            ival, tz, ambiguous="raise", nonexistent=None, creso=reso
+                        )
+                    obj.value = ival
+                    maybe_localize_tso(obj, tz, obj.creso)
+                    return obj
 
         dt = parse_datetime_string(
             ts,

pandas/_libs/tslibs/parsing.pyx

@@ -377,32 +377,33 @@ def parse_datetime_string_with_reso(
         raise ValueError(f'Given date string "{date_string}" not likely a datetime')
 
     # Try iso8601 first, as it handles nanoseconds
-    string_to_dts_failed = string_to_dts(
-        date_string, &dts, &out_bestunit, &out_local,
-        &out_tzoffset, False
-    )
-    if not string_to_dts_failed:
-        # Match Timestamp and drop picoseconds, femtoseconds, attoseconds
-        # The new resolution will just be nano
-        # GH#50417
-        if out_bestunit in _timestamp_units:
-            out_bestunit = NPY_DATETIMEUNIT.NPY_FR_ns
-
-        if out_bestunit == NPY_DATETIMEUNIT.NPY_FR_ns:
-            # TODO: avoid circular import
-            from pandas import Timestamp
-            parsed = Timestamp(date_string)
-        else:
-            if out_local:
-                tz = timezone(timedelta(minutes=out_tzoffset))
+    if not dayfirst:  # GH 58859
+        string_to_dts_failed = string_to_dts(
+            date_string, &dts, &out_bestunit, &out_local,
+            &out_tzoffset, False
+        )
+        if not string_to_dts_failed:
+            # Match Timestamp and drop picoseconds, femtoseconds, attoseconds
+            # The new resolution will just be nano
+            # GH#50417
+            if out_bestunit in _timestamp_units:
+                out_bestunit = NPY_DATETIMEUNIT.NPY_FR_ns
+
+            if out_bestunit == NPY_DATETIMEUNIT.NPY_FR_ns:
+                # TODO: avoid circular import
+                from pandas import Timestamp
+                parsed = Timestamp(date_string)
             else:
-                tz = None
-            parsed = datetime_new(
-                dts.year, dts.month, dts.day, dts.hour, dts.min, dts.sec, dts.us, tz
-            )
+                if out_local:
+                    tz = timezone(timedelta(minutes=out_tzoffset))
+                else:
+                    tz = None
+                parsed = datetime_new(
+                    dts.year, dts.month, dts.day, dts.hour, dts.min, dts.sec, dts.us, tz
+                )
 
-        reso = npy_unit_to_attrname[out_bestunit]
-        return parsed, reso
+            reso = npy_unit_to_attrname[out_bestunit]
+            return parsed, reso
 
     parsed = _parse_delimited_date(date_string, dayfirst, &out_bestunit)
     if parsed is not None:

pandas/tests/tools/test_to_datetime.py

@@ -2988,6 +2988,8 @@ def test_parsers_nat(self):
             ("20/12/21", True, False, datetime(2021, 12, 20)),
             ("20/12/21", False, True, datetime(2020, 12, 21)),
             ("20/12/21", True, True, datetime(2020, 12, 21)),
+            # GH 58859
+            ("20201012", True, False, datetime(2020, 12, 10)),
         ],
     )
     def test_parsers_dayfirst_yearfirst(