DEPR: inconsistent string parsing in DatetimeIndex.indexer_at_time

doc/source/whatsnew/v3.0.0.rst

@@ -737,6 +737,7 @@ Other Deprecations
 - Deprecated using ``epoch`` date format in :meth:`DataFrame.to_json` and :meth:`Series.to_json`, use ``iso`` instead. (:issue:`57063`)
 - Deprecated allowing ``fill_value`` that cannot be held in the original dtype (excepting NA values for integer and bool dtypes) in :meth:`Series.unstack` and :meth:`DataFrame.unstack` (:issue:`12189`, :issue:`53868`)
 - 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 option "future.no_silent_downcasting", as it is no longer used. In a future version accessing this option will raise (:issue:`59502`)
 - Deprecated silent casting of non-datetime 'other' to datetime in :meth:`Series.combine_first` (:issue:`62931`)

pandas/core/indexes/datetimes.py

@@ -767,7 +767,37 @@ def indexer_at_time(self, time, asof: bool = False) -> npt.NDArray[np.intp]:
         if isinstance(time, str):
             from dateutil.parser import parse
 
-            time = parse(time).time()
+            orig = time
+            try:
+                alt = to_time(time)
+            except ValueError:
+                warnings.warn(
+                    # GH#50839
+                    f"The string '{orig}' cannot be parsed using pd.core.tools.to_time "
+                    f"and in a future version will raise. "
+                    "Use an unambiguous time string format or explicitly cast to "
+                    "`datetime.time` before calling.",
+                    Pandas4Warning,
+                    stacklevel=find_stack_level(),
+                )
+                time = parse(time).time()
+            else:
+                try:
+                    time = parse(time).time()
+                except ValueError:
+                    # e.g. '23550' raises dateutil.parser._parser.ParserError
+                    time = alt
+                if alt != time:
+                    warnings.warn(
+                        # GH#50839
+                        f"The string '{orig}' is currently parsed as {time} "
+                        f"but in a future version will be parsed as {alt}, consistent"
+                        "with `between_time` behavior. To avoid this warning, "
+                        "use an unambiguous string format or explicitly cast to "
+                        "`datetime.time` before calling.",
+                        Pandas4Warning,
+                        stacklevel=find_stack_level(),
+                    )
 
         if time.tzinfo:
             if self.tz is None:

pandas/tests/frame/methods/test_at_time.py

@@ -8,6 +8,7 @@
 import pytest
 
 from pandas._libs.tslibs import timezones
+from pandas.errors import Pandas4Warning
 
 from pandas import (
     DataFrame,
@@ -132,3 +133,21 @@ def test_at_time_datetimeindex(self):
         tm.assert_frame_equal(result, expected)
         tm.assert_frame_equal(result, expected2)
         assert len(result) == 4
+
+    def test_at_time_ambiguous_format_deprecation(self):
+        # GH#50839
+        rng = date_range("1/1/2000", "1/5/2000", freq="125min")
+        ts = DataFrame(list(range(len(rng))), index=rng)
+
+        msg1 = "The string '.*' cannot be parsed"
+        with tm.assert_produces_warning(Pandas4Warning, match=msg1):
+            ts.at_time("2022-12-12 00:00:00")
+        with tm.assert_produces_warning(Pandas4Warning, match=msg1):
+            ts.at_time("2022-12-12 00:00:00 +09:00")
+        with tm.assert_produces_warning(Pandas4Warning, match=msg1):
+            ts.at_time("2022-12-12 00:00:00.000000")
+
+        # The dateutil parser raises on these, so we can give the future behavior
+        #  immediately using pd.core.tools.to_time
+        ts.at_time("235500")
+        ts.at_time("115500PM")

pandas/tests/generic/test_finalize.py

@@ -3,6 +3,7 @@
 """
 
 from copy import deepcopy
+from datetime import time
 import operator
 import re
 
@@ -280,12 +281,12 @@
     (
         pd.Series,
         (1, pd.date_range("2000", periods=4)),
-        operator.methodcaller("at_time", "12:00"),
+        operator.methodcaller("at_time", time(12)),
     ),
     (
         pd.DataFrame,
         ({"A": [1, 1, 1, 1]}, pd.date_range("2000", periods=4)),
-        operator.methodcaller("at_time", "12:00"),
+        operator.methodcaller("at_time", time(12)),
     ),
     (
         pd.Series,