DEPR: passing mixed offsets with utc=False into to_datetime

doc/source/whatsnew/v0.24.0.rst

@@ -643,7 +643,7 @@ Parsing datetime strings with different UTC offsets will now create an Index of
 Passing ``utc=True`` will mimic the previous behavior but will correctly indicate
 that the dates have been converted to UTC
 
-.. ipython:: python
+.. code-block:: ipython
 
     pd.to_datetime(["2015-11-18 15:30:00+05:30",
                     "2015-11-18 16:30:00+06:30"], utc=True)

pandas/_libs/tslib.pyx

@@ -561,14 +561,6 @@ cpdef array_to_datetime(
         #    (with individual dateutil.tzoffsets) are returned
         is_same_offsets = len(out_tzoffset_vals) == 1
         if not is_same_offsets:
-            warnings.warn(
-                "In a future version of pandas, parsing datetimes with mixed time "
-                "zones will raise a warning unless `utc=True`. "
-                "Please specify `utc=True to opt in to the new behaviour "
-                "and silence this warning.",
-                FutureWarning,
-                stacklevel=find_stack_level(),
-            )
             return _array_to_datetime_object(values, errors, dayfirst, yearfirst)
         else:
             tz_offset = out_tzoffset_vals.pop()
@@ -628,6 +620,7 @@ cdef _array_to_datetime_object(
     # 1) NaT or NaT-like values
     # 2) datetime strings, which we return as datetime.datetime
     # 3) special strings - "now" & "today"
+    unique_timezones = set()
     for i in range(n):
         # Analogous to: val = values[i]
         val = <object>(<PyObject**>cnp.PyArray_MultiIter_DATA(mi, 1))[0]
@@ -657,6 +650,7 @@ cdef _array_to_datetime_object(
                     tzinfo=tsobj.tzinfo,
                     fold=tsobj.fold,
                 )
+                unique_timezones.add(tsobj.tzinfo)
 
             except (ValueError, OverflowError) as ex:
                 ex.args = (f"{ex}, at position {i}", )
@@ -674,6 +668,15 @@ cdef _array_to_datetime_object(
 
         cnp.PyArray_MultiIter_NEXT(mi)
 
+    if len(unique_timezones) > 1:
+        warnings.warn(
+            "In a future version of pandas, parsing datetimes with mixed time "
+            "zones will raise a warning unless `utc=True`. "
+            "Please specify `utc=True to opt in to the new behaviour "
+            "and silence this warning.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
     return oresult_nd, None
 
 

pandas/tests/tools/test_to_datetime.py

@@ -3680,3 +3680,27 @@ def test_from_numeric_arrow_dtype(any_numeric_ea_dtype):
     result = to_datetime(ser)
     expected = Series([1, 2], dtype="datetime64[ns]")
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "date, date_expected, warning",
+    [
+        (["2020-01-01 00:00+00:00", "2020-01-01 00:00+02:00", ""], None, FutureWarning),
+        (
+            ["2020-01-01 00:00+00:00", ""],
+            [Timestamp("2020-01-01 00:00+00:00"), "NaT"],
+            None,
+        ),
+    ],
+)
+def test_to_datetime_with_empty_str_utcFalse_format_mixed(date, date_expected, warning):
+    msg = "In a future version of pandas, parsing datetimes with mixed time zones "
+    "will raise a warning unless `utc=True`."
+
+    if warning is not None:
+        with tm.assert_produces_warning(warning, match=msg):
+            to_datetime(date, format="mixed")
+    else:
+        result = to_datetime(date, format="mixed")
+        expected = Index(date_expected, dtype=object)
+        tm.assert_index_equal(result, expected)