DEPR: Deprecate non-ISO date string formats in DatetimeIndex.loc

doc/source/whatsnew/v3.0.0.rst

@@ -739,6 +739,7 @@ Other Deprecations
 - Deprecated allowing ``fill_value`` that cannot be held in the original dtype (excepting NA values for integer and bool dtypes) in :meth:`Series.shift` and :meth:`DataFrame.shift` (:issue:`53802`)
 - Deprecated allowing strings representing full dates in :meth:`DataFrame.at_time` and :meth:`Series.at_time` (:issue:`50839`)
 - Deprecated backward-compatibility behavior for :meth:`DataFrame.select_dtypes` matching "str" dtype when ``np.object_`` is specified (:issue:`61916`)
+- Deprecated non-ISO date string formats in :meth:`DatetimeIndex.__getitem__` with string labels. Use ISO format (YYYY-MM-DD) instead. (:issue:`58302`)
 - Deprecated option "future.no_silent_downcasting", as it is no longer used. In a future version accessing this option will raise (:issue:`59502`)
 - Deprecated passing non-Index types to :meth:`Index.join`; explicitly convert to Index first (:issue:`62897`)
 - Deprecated silent casting of non-datetime 'other' to datetime in :meth:`Series.combine_first` (:issue:`62931`)

pandas/core/indexes/datetimes.py

@@ -2,6 +2,7 @@
 
 import datetime as dt
 import operator
+import re
 from typing import (
     TYPE_CHECKING,
     Self,
@@ -110,6 +111,28 @@ def _new_DatetimeIndex(cls, d):
     return result
 
 
+def _is_iso_format_string(date_str: str) -> bool:
+    """
+    Check if a date string follows ISO8601 format.
+
+    ISO format must start with a 4-digit year (YYYY), optionally followed by
+    month and day with consistent separators.
+
+    Examples of ISO format (True):
+    - 2024-01-10
+    - 2024/01/10
+    - 2024 01 10
+    - 2024-01-10T00:00:00
+
+    Examples of non-ISO format (False):
+    - 01/10/2024 (MM/DD/YYYY)
+    - 10/01/2024 (DD/MM/YYYY)
+    - 01-10-2024 (MM-DD-YYYY)
+    """
+    # ISO format must start with 4-digit year followed by separator (-, /, ., or space)
+    return re.match(r"^\d{4}[-/. ]", date_str) is not None
+
+
 @inherit_names(
     DatetimeArray._field_ops
     + [
@@ -613,6 +636,14 @@ def get_loc(self, key):
                 parsed, reso = self._parse_with_reso(key)
             except ValueError as err:
                 raise KeyError(key) from err
+            # GH#58302 - Deprecate non-ISO string formats in .loc indexing
+            if not _is_iso_format_string(key):
+                msg = (
+                    "Parsing non-ISO datetime strings in .loc is deprecated "
+                    "and will be removed in a future version. Use ISO format "
+                    f"(YYYY-MM-DD) instead. Got '{key}'."
+                )
+                warnings.warn(msg, Pandas4Warning, stacklevel=find_stack_level())
             self._disallow_mismatched_indexing(parsed)
 
             if self._can_partial_date_slice(reso):
@@ -702,11 +733,31 @@ def check_str_or_none(point) -> bool:
         in_index = True
         if start is not None:
             start_casted = self._maybe_cast_slice_bound(start, "left")
+
+            # GH#58302 - Deprecate non-ISO string formats in .loc indexing
+            if isinstance(start, str) and not _is_iso_format_string(start):
+                msg = (
+                    "Parsing non-ISO datetime strings in .loc is deprecated "
+                    "and will be removed in a future version. Use ISO format "
+                    f"(YYYY-MM-DD) instead. Got '{start}'."
+                )
+                warnings.warn(msg, Pandas4Warning, stacklevel=find_stack_level())
+
             mask = start_casted <= self
             in_index &= (start_casted == self).any()
 
         if end is not None:
             end_casted = self._maybe_cast_slice_bound(end, "right")
+
+            # GH#58302 - Deprecate non-ISO string formats in .loc indexing
+            if isinstance(end, str) and not _is_iso_format_string(end):
+                msg = (
+                    "Parsing non-ISO datetime strings in .loc is deprecated "
+                    "and will be removed in a future version. Use ISO format "
+                    f"(YYYY-MM-DD) instead. Got '{end}'."
+                )
+                warnings.warn(msg, Pandas4Warning, stacklevel=find_stack_level())
+
             mask = (self <= end_casted) & mask
             in_index &= (end_casted == self).any()
 

pandas/tests/groupby/test_groupby.py

@@ -28,7 +28,12 @@
 from pandas.core.arrays import BooleanArray
 import pandas.core.common as com
 
-pytestmark = pytest.mark.filterwarnings("ignore:Mean of empty slice:RuntimeWarning")
+pytestmark = [
+    pytest.mark.filterwarnings("ignore:Mean of empty slice:RuntimeWarning"),
+    pytest.mark.filterwarnings(
+        "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+    ),
+]
 
 
 def test_repr():

pandas/tests/indexes/datetimes/test_indexing.py

@@ -26,6 +26,10 @@
 
 from pandas.tseries.frequencies import to_offset
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 START, END = datetime(2009, 1, 1), datetime(2010, 1, 1)
 
 

pandas/tests/indexes/datetimes/test_partial_slicing.py

@@ -5,6 +5,8 @@
 import numpy as np
 import pytest
 
+from pandas.errors import Pandas4Warning
+
 from pandas import (
     DataFrame,
     DatetimeIndex,
@@ -19,6 +21,10 @@
 
 
 class TestSlicing:
+    pytestmark = pytest.mark.filterwarnings(
+        "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+    )
+
     def test_string_index_series_name_converted(self):
         # GH#1644
         df = DataFrame(
@@ -464,3 +470,96 @@ def test_slice_reduce_to_series(self):
         )
         result = df.loc["2000", "A"]
         tm.assert_series_equal(result, expected)
+
+
+class TestDatetimeIndexNonISODeprecation:
+    """Tests for deprecation of non-ISO string formats in .loc indexing. GH#58302"""
+
+    @pytest.fixture
+    def ser_daily(self):
+        """Create a Series with daily DatetimeIndex for testing."""
+        return Series(
+            range(15),
+            index=DatetimeIndex(date_range(start="2024-01-01", freq="D", periods=15)),
+        )
+
+    @pytest.mark.parametrize(
+        "date_string",
+        [
+            "1/10/2024",  # MM/DD/YYYY format
+            "01/10/2024",  # MM/DD/YYYY format with leading zero
+        ],
+    )
+    def test_loc_indexing_non_iso_single_key_deprecation(self, ser_daily, date_string):
+        # GH#58302
+        msg = "Parsing non-ISO datetime strings in .loc is deprecated"
+
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            result = ser_daily.loc[date_string]
+            assert result == 9
+
+    @pytest.mark.parametrize(
+        "date_string,expected",
+        [
+            ("2024-01-10", 9),  # YYYY-MM-DD (dash)
+            ("2024/01/10", 9),  # YYYY/MM/DD (slash)
+            ("2024 01 10", 9),  # YYYY MM DD (space)
+        ],
+    )
+    def test_loc_indexing_iso_format_no_warning(self, ser_daily, date_string, expected):
+        # GH#58302 - ISO formats should NOT warn
+        with tm.assert_produces_warning(None):
+            result = ser_daily.loc[date_string]
+            assert result == expected
+
+    @pytest.mark.parametrize(
+        "start_string",
+        [
+            "1/10/2024",  # MM/DD/YYYY format
+            "01/10/2024",  # MM/DD/YYYY format with leading zero
+        ],
+    )
+    def test_loc_slicing_non_iso_start_deprecation(self, ser_daily, start_string):
+        # GH#58302 - Non-ISO start in slice should warn
+        msg = "Parsing non-ISO datetime strings in .loc is deprecated"
+
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            result = ser_daily.loc[start_string:"2024-01-15"]
+            assert len(result) > 0
+
+    @pytest.mark.parametrize(
+        "end_string",
+        [
+            "5-01-2024",  # DD-MM-YYYY format
+            "05-01-2024",  # DD-MM-YYYY format with leading zero
+        ],
+    )
+    def test_loc_slicing_non_iso_end_deprecation(self, ser_daily, end_string):
+        # GH#58302 - Non-ISO end in slice should warn
+        msg = "Parsing non-ISO datetime strings in .loc is deprecated"
+
+        with tm.assert_produces_warning(Pandas4Warning, match=msg):
+            result = ser_daily.loc["2024-01-01":end_string]
+            assert len(result) > 0
+
+    def test_loc_slicing_both_non_iso_deprecation(self, ser_daily):
+        # GH#58302 - Both non-ISO should warn (twice)
+        msg = "Parsing non-ISO datetime strings in .loc is deprecated"
+
+        with tm.assert_produces_warning(
+            Pandas4Warning, match=msg, check_stacklevel=False
+        ):
+            result = ser_daily.loc["1/10/2024":"5-01-2024"]
+            assert len(result) > 0
+
+    def test_loc_slicing_iso_formats_no_warning(self, ser_daily):
+        # GH#58302 - ISO slice formats should NOT warn
+        with tm.assert_produces_warning(None):
+            result = ser_daily.loc["2024-01-05":"2024-01-10"]
+            assert len(result) == 6
+
+    def test_loc_non_string_keys_no_warning(self, ser_daily):
+        # GH#58302 - Non-string keys should not warn
+        with tm.assert_produces_warning(None):
+            result = ser_daily.loc[Timestamp("2024-01-10")]
+            assert result == 9

pandas/tests/indexes/multi/test_partial_indexing.py

@@ -9,6 +9,10 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 @pytest.fixture
 def df():

pandas/tests/indexes/period/test_indexing.py

@@ -21,6 +21,10 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 dti4 = date_range("2016-01-01", periods=4)
 dti = dti4[:-1]
 rng = pd.Index(range(3))

pandas/tests/indexes/period/test_partial_slicing.py

@@ -10,6 +10,10 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 class TestPeriodIndex:
     def test_getitem_periodindex_duplicates_string_slice(self):

pandas/tests/indexing/multiindex/test_slice.py

@@ -19,6 +19,10 @@
 import pandas._testing as tm
 from pandas.tests.indexing.common import _mklbl
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 class TestMultiIndexSlicers:
     def test_per_axis_per_level_getitem(self):

pandas/tests/indexing/test_loc.py

@@ -44,6 +44,10 @@
 from pandas.core.indexing import _one_ellipsis_message
 from pandas.tests.indexing.common import check_indexing_smoketest_or_raises
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 @pytest.mark.parametrize(
     "series, new_series, expected_ser",

pandas/tests/resample/test_datetime_index.py

@@ -38,6 +38,10 @@
 from pandas.tseries.frequencies import to_offset
 from pandas.tseries.offsets import Minute
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 @pytest.fixture
 def simple_date_range_series():

pandas/tests/resample/test_resampler_grouper.py

@@ -16,6 +16,10 @@
 import pandas._testing as tm
 from pandas.core.indexes.datetimes import date_range
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 @pytest.fixture
 def test_frame():

pandas/tests/series/indexing/test_datetime.py

@@ -27,6 +27,10 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 def test_fancy_getitem():
     dti = date_range(

pandas/tests/series/indexing/test_getitem.py

@@ -37,6 +37,10 @@
 
 from pandas.tseries.offsets import BDay
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 class TestSeriesGetitemScalars:
     def test_getitem_object_index_float_string(self):

pandas/tests/series/indexing/test_xs.py

@@ -8,6 +8,10 @@
 )
 import pandas._testing as tm
 
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:Parsing non-ISO datetime strings:pandas.errors.Pandas4Warning"
+)
+
 
 def test_xs_datetimelike_wrapping():
     # GH#31630 a case where we shouldn't wrap datetime64 in Timestamp