DEPR: passing mixed offsets with utc=False into to_datetime

doc/source/user_guide/io.rst

@@ -935,6 +935,8 @@ Parsing a CSV with mixed timezones
 pandas cannot natively represent a column or index with mixed timezones. If your CSV
 file contains columns with a mixture of timezones, the default result will be
 an object-dtype column with strings, even with ``parse_dates``.
+To parse the mixed-timezone values as a datetime column, read in as ``object`` dtype and
+then call :func:`to_datetime` with ``utc=True``.
 
 
 .. ipython:: python
@@ -943,14 +945,6 @@ an object-dtype column with strings, even with ``parse_dates``.
    a
    2000-01-01T00:00:00+05:00
    2000-01-01T00:00:00+06:00"""
-   df = pd.read_csv(StringIO(content), parse_dates=["a"])
-   df["a"]
-
-To parse the mixed-timezone values as a datetime column, read in as ``object`` dtype and
-then call :func:`to_datetime` with ``utc=True``.
-
-.. ipython:: python
-
    df = pd.read_csv(StringIO(content))
    df["a"] = pd.to_datetime(df["a"], utc=True)
    df["a"]

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

@@ -620,6 +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]
@@ -649,6 +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}", )
@@ -666,6 +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/core/tools/datetimes.py

@@ -337,6 +337,8 @@ def _return_parsed_timezone_results(
     tz_result : Index-like of parsed dates with timezone
     """
     tz_results = np.empty(len(result), dtype=object)
+    unique(timezones)
+    non_na_timezones = set()
     for zone in unique(timezones):
         mask = timezones == zone
         dta = DatetimeArray(result[mask]).tz_localize(zone)
@@ -345,8 +347,18 @@ def _return_parsed_timezone_results(
                 dta = dta.tz_localize("utc")
             else:
                 dta = dta.tz_convert("utc")
+        else:
+            if not dta.isna().all():
+                non_na_timezones.add(zone)
         tz_results[mask] = dta
-
+    if len(non_na_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 Index(tz_results, name=name)
 
 

pandas/tests/indexes/datetimes/test_constructors.py

@@ -301,7 +301,10 @@ def test_construction_index_with_mixed_timezones(self):
 
         msg = "DatetimeIndex has mixed timezones"
         with pytest.raises(TypeError, match=msg):
-            DatetimeIndex(["2013-11-02 22:00-05:00", "2013-11-03 22:00-06:00"])
+            msg = "In a future version of pandas, parsing datetimes with mixed "
+            "time zones will raise a warning unless `utc=True`."
+            with tm.assert_produces_warning(FutureWarning, match=msg):
+                DatetimeIndex(["2013-11-02 22:00-05:00", "2013-11-03 22:00-06:00"])
 
         # length = 1
         result = Index([Timestamp("2011-01-01")], name="idx")

pandas/tests/io/json/test_readlines.py

@@ -54,7 +54,10 @@ def test_read_datetime(request, engine):
     if engine == "pyarrow":
         result = read_json(StringIO(json_line), engine=engine)
     else:
-        result = read_json(StringIO(json_line), engine=engine)
+        msg = "In a future version of pandas, parsing datetimes with mixed time zones "
+        "will raise a warning unless `utc=True`."
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = read_json(StringIO(json_line), engine=engine)
     expected = DataFrame(
         [[1, "2020-03-05", "hector"], [2, "2020-04-08T09:58:49+00:00", "hector"]],
         columns=["accounts", "date", "name"],