undo unnecessary adjustments

Author: MarcoGorelli

doc/source/user_guide/timedeltas.rst

@@ -424,6 +424,7 @@ Similarly to other of the datetime-like indices, ``DatetimeIndex`` and ``PeriodI
 Selections work similarly, with coercion on string-likes and slices:
 
 .. ipython:: python
+   :okwarning:
 
    s["1 day":"2 day"]
    s["1 day 01:00:00"]
@@ -432,6 +433,7 @@ Selections work similarly, with coercion on string-likes and slices:
 Furthermore you can use partial string selection and the range will be inferred:
 
 .. ipython:: python
+   :okwarning:
 
    s["1 day":"1 day 5 hours"]
 

pandas/_libs/tslibs/parsing.pyx

@@ -263,6 +263,17 @@ cdef inline bint does_string_look_like_time(str parse_string):
 
     return 0 <= hour <= 23 and 0 <= minute <= 59
 
+from pandas.util._exceptions import find_stack_level
+
+
+def du_parse_with_warning(*args, **kwargs):
+    parsed = du_parse(*args, **kwargs)
+    warnings.warn(
+        "Parsing datetime strings without a format specified, "
+        "please specify a format to avoid unexpected results",
+        stacklevel=find_stack_level(),
+    )
+
 
 def parse_datetime_string(
     # NB: This will break with np.str_ (GH#32264) even though
@@ -290,8 +301,12 @@ def parse_datetime_string(
 
     if does_string_look_like_time(date_string):
         # use current datetime as default, not pass _DEFAULT_DATETIME
-        dt = du_parse(date_string, dayfirst=dayfirst,
-                      yearfirst=yearfirst, **kwargs)
+        dt = du_parse_with_warning(
+            date_string,
+            dayfirst=dayfirst,
+            yearfirst=yearfirst,
+            **kwargs,
+        )
         return dt
 
     dt, _ = _parse_delimited_date(date_string, dayfirst)
@@ -307,8 +322,13 @@ def parse_datetime_string(
         pass
 
     try:
-        dt = du_parse(date_string, default=_DEFAULT_DATETIME,
-                      dayfirst=dayfirst, yearfirst=yearfirst, **kwargs)
+        dt = du_parse_with_warning(
+            date_string,
+            default=_DEFAULT_DATETIME,
+            dayfirst=dayfirst,
+            yearfirst=yearfirst,
+            **kwargs,
+        )
     except TypeError:
         # following may be raised from dateutil
         # TypeError: 'NoneType' object is not iterable
@@ -706,7 +726,11 @@ def try_parse_dates(
             date = datetime.now()
             default = datetime(date.year, date.month, 1)
 
-        parse_date = lambda x: du_parse(x, dayfirst=dayfirst, default=default)
+        parse_date = lambda x: du_parse_with_warning(
+            x,
+            dayfirst=dayfirst,
+            default=default,
+        )
 
         # EAFP here
         try:
@@ -753,13 +777,17 @@ def try_parse_date_and_time(
             date = datetime.now()
             default = datetime(date.year, date.month, 1)
 
-        parse_date = lambda x: du_parse(x, dayfirst=dayfirst, default=default)
+        parse_date = lambda x: du_parse_with_warning(
+            x,
+            dayfirst=dayfirst,
+            default=default,
+        )
 
     else:
         parse_date = date_parser
 
     if time_parser is None:
-        parse_time = lambda x: du_parse(x)
+        parse_time = lambda x: du_parse_with_warning(x)
 
     else:
         parse_time = time_parser
@@ -980,7 +1008,7 @@ def guess_datetime_format(dt_str, bint dayfirst=False):
         datetime_attrs_to_format.insert(0, day_attribute_and_format)
 
     try:
-        parsed_datetime = du_parse(dt_str, dayfirst=dayfirst)
+        parsed_datetime = du_parse_with_warning(dt_str, dayfirst=dayfirst)
     except (ValueError, OverflowError):
         # In case the datetime can't be parsed, its format cannot be guessed
         return None

pandas/tests/frame/methods/test_reset_index.py

@@ -337,24 +337,19 @@ def test_reset_index_multiindex_nan(self):
         tm.assert_frame_equal(rs, df)
 
     @pytest.mark.parametrize(
-        "name",
+        "name, warn",
         [
-            None,
-            "foo",
-            2,
-            3.0,
-            pd.Timedelta(6),
-            Timestamp("2012-12-30", tz="UTC"),
-            "2012-12-31",
+            (None, UserWarning),
+            ("foo", UserWarning),
+            (2, None),
+            (3.0, None),
+            (pd.Timedelta(6), None),
+            (Timestamp("2012-12-30", tz="UTC"), FutureWarning),
+            ("2012-12-31", None),
         ],
     )
-    def test_reset_index_with_datetimeindex_cols(self, name):
+    def test_reset_index_with_datetimeindex_cols(self, name, warn):
         # GH#5818
-        warn = None
-        if isinstance(name, Timestamp) and name.tz is not None:
-            # _deprecate_mismatched_indexing
-            warn = FutureWarning
-
         df = DataFrame(
             [[1, 2], [3, 4]],
             columns=date_range("1/1/2013", "1/2/2013"),