BUG: partially-inferring pydatetime objects

doc/source/whatsnew/v1.4.0.rst

@@ -475,6 +475,7 @@ Datetimelike
 - Bug in :meth:`date_range` and :meth:`bdate_range` do not return right bound when ``start`` = ``end`` and set is closed on one side (:issue:`43394`)
 - Bug in inplace addition and subtraction of :class:`DatetimeIndex` or :class:`TimedeltaIndex` with :class:`DatetimeArray` or :class:`TimedeltaArray` (:issue:`43904`)
 - Bug in in calling ``np.isnan``, ``np.isfinite``, or ``np.isinf`` on a timezone-aware :class:`DatetimeIndex` incorrectly raising ``TypeError`` (:issue:`43917`)
+- Bug in constructing a :class:`Series` from datetime-like strings with mixed timezones incorrectly partially-inferring datetime values (:issue:`40111`)
 -
 
 Timedelta

pandas/core/dtypes/cast.py

@@ -1526,7 +1526,7 @@ def try_datetime(v: np.ndarray) -> ArrayLike:
         try:
             # GH#19671 we pass require_iso8601 to be relatively strict
             #  when parsing strings.
-            dta = sequence_to_datetimes(v, require_iso8601=True, allow_object=True)
+            dta = sequence_to_datetimes(v, require_iso8601=True, allow_object=False)
         except (ValueError, TypeError):
             # e.g. <class 'numpy.timedelta64'> is not convertible to datetime
             return v.reshape(shape)

pandas/tests/series/test_constructors.py

@@ -1045,6 +1045,18 @@ def test_constructor_with_datetime_tz2(self):
         expected = Series(DatetimeIndex(["NaT", "NaT"], tz="US/Eastern"))
         tm.assert_series_equal(s, expected)
 
+    def test_constructor_no_partial_datetime_casting(self):
+        # GH#40111
+        vals = [
+            "nan",
+            Timestamp("1990-01-01"),
+            "2015-03-14T16:15:14.123-08:00",
+            "2019-03-04T21:56:32.620-07:00",
+            None,
+        ]
+        ser = Series(vals)
+        assert all(ser[i] is vals[i] for i in range(len(vals)))
+
     @pytest.mark.parametrize("arr_dtype", [np.int64, np.float64])
     @pytest.mark.parametrize("dtype", ["M8", "m8"])
     @pytest.mark.parametrize("unit", ["ns", "us", "ms", "s", "h", "m", "D"])

pandas/tests/tools/test_to_datetime.py

@@ -1123,17 +1123,32 @@ def test_iso8601_strings_mixed_offsets_with_naive(self):
 
     def test_mixed_offsets_with_native_datetime_raises(self):
         # GH 25978
-        ser = Series(
+
+        vals = [
+            "nan",
+            Timestamp("1990-01-01"),
+            "2015-03-14T16:15:14.123-08:00",
+            "2019-03-04T21:56:32.620-07:00",
+            None,
+        ]
+        ser = Series(vals)
+        assert all(ser[i] is vals[i] for i in range(len(vals)))  # GH#40111
+
+        mixed = to_datetime(ser)
+        expected = Series(
             [
-                "nan",
+                "NaT",
                 Timestamp("1990-01-01"),
-                "2015-03-14T16:15:14.123-08:00",
-                "2019-03-04T21:56:32.620-07:00",
+                Timestamp("2015-03-14T16:15:14.123-08:00").to_pydatetime(),
+                Timestamp("2019-03-04T21:56:32.620-07:00").to_pydatetime(),
                 None,
-            ]
+            ],
+            dtype=object,
         )
+        tm.assert_series_equal(mixed, expected)
+
         with pytest.raises(ValueError, match="Tz-aware datetime.datetime"):
-            to_datetime(ser)
+            to_datetime(mixed)
 
     def test_non_iso_strings_with_tz_offset(self):
         result = to_datetime(["March 1, 2018 12:00:00+0400"] * 2)