Reject tz-aware datetime / time when utcoffset() returns None

The datetime / time ISO 8601 encoders previously fell through to the
naive format when an aware value's tzinfo subclass returned ``None``
from ``utcoffset(...)``. The branch was conservatively documented as
"broken tzinfo subclass" but the silent demotion lost the user's
tz-awareness intent without warning, and was asymmetric with the
sibling ``_format_utc_offset`` rejection behaviour.

Raise ``DataError`` in both branches so the caller sees the broken
tzinfo contract immediately. Update the broken-tzinfo encoder pin
tests to assert the new rejection contract; drop the silent-naive
round-trip pins.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqlitedbapi/types.py

@@ -333,10 +333,16 @@ def _iso8601_from_datetime(value: datetime.datetime | datetime.date) -> str:
         if value.tzinfo is None:
             return base
         # tzinfo is set (checked above), so utcoffset() returns timedelta.
-        # A None here would indicate a broken tzinfo subclass; be explicit.
+        # A None here means the tzinfo subclass declared itself but
+        # cannot resolve an offset for this datetime — be explicit
+        # and reject rather than silently demoting to naive (which
+        # would lose the user's tz-awareness intent without warning).
         offset = value.utcoffset()
         if offset is None:
-            return base
+            raise DataError(
+                f"datetime is tz-aware but tzinfo.utcoffset() returned None for "
+                f"{value!r}; cannot encode without a resolvable UTC offset"
+            )
         return base + _format_utc_offset(offset)
     # datetime.date (must come after datetime check — datetime is a subclass).
     return value.isoformat()
@@ -359,7 +365,10 @@ def _iso8601_from_time(value: datetime.time) -> str:
         return base
     offset = value.utcoffset()
     if offset is None:
-        return base
+        raise DataError(
+            f"time is tz-aware but tzinfo.utcoffset() returned None for "
+            f"{value!r}; cannot encode without a resolvable UTC offset"
+        )
     return base + _format_utc_offset(offset)
 
 

tests/test_iso8601_encoder.py

@@ -204,61 +204,30 @@ def tzname(self, dt: datetime.datetime | None) -> str:
 
 
 class TestIso8601EncoderBrokenTzinfo:
-    """Pin the ``utcoffset() is None`` fallback branch on both encoders.
-
-    Sibling coverage: naive / aware-with-positive-offset / aware-with-
-    negative-offset / aware-with-sub-minute-offset / microseconds are
-    all already pinned above. The missing case is an aware value whose
-    tzinfo declines to produce an offset — the encoder must degrade to
-    the naive format, not crash.
+    """Pin the ``utcoffset() is None`` rejection branch on both encoders.
+
+    A tzinfo subclass that declares itself but cannot resolve an
+    offset for the given datetime/time is a broken contract. Cycle 22
+    flipped this from silent demotion (encoded as naive, losing the
+    user's tz-awareness intent) to a hard ``DataError``. Pin the new
+    contract so a regression that re-introduces silent demotion is
+    caught.
     """
 
-    def test_datetime_broken_tzinfo_returns_naive_format(self) -> None:
-        value = datetime.datetime(2024, 5, 1, 12, 30, 45, tzinfo=_AbstractTz())
-        assert _iso8601_from_datetime(value) == "2024-05-01 12:30:45"
-
-    def test_datetime_broken_tzinfo_preserves_microseconds(self) -> None:
-        # Pins the ordering of the microsecond append and the
-        # offset-fallback branch. A regression that swapped the two
-        # would fail here but not in the bare-naive test above.
-        value = datetime.datetime(2024, 5, 1, 12, 30, 45, 123456, tzinfo=_AbstractTz())
-        assert _iso8601_from_datetime(value) == "2024-05-01 12:30:45.123456"
-
-    def test_datetime_broken_tzinfo_round_trips_as_naive(self) -> None:
-        # Ties the "abstract tzinfo degrades to naive" encoder contract
-        # to the decode side: feed the encoded text back through the
-        # decoder and confirm the result has no tzinfo. A future
-        # regression that emitted "+00:00" for broken tzinfo would
-        # decode back to an aware UTC datetime and fail this pin.
-        from dqlitedbapi.types import _datetime_from_iso8601
+    def test_datetime_broken_tzinfo_raises_data_error(self) -> None:
+        from dqlitedbapi.exceptions import DataError
 
-        original = datetime.datetime(2024, 5, 1, 12, 30, 45, 123456, tzinfo=_AbstractTz())
-        encoded = _iso8601_from_datetime(original)
-        decoded = _datetime_from_iso8601(encoded)
-        assert decoded is not None
-        assert decoded.tzinfo is None
-        assert decoded == datetime.datetime(2024, 5, 1, 12, 30, 45, 123456)
+        value = datetime.datetime(2024, 5, 1, 12, 30, 45, tzinfo=_AbstractTz())
+        with pytest.raises(DataError, match="utcoffset"):
+            _iso8601_from_datetime(value)
 
-    def test_time_broken_tzinfo_returns_naive_format(self) -> None:
+    def test_time_broken_tzinfo_raises_data_error(self) -> None:
+        from dqlitedbapi.exceptions import DataError
         from dqlitedbapi.types import _iso8601_from_time
 
         value = datetime.time(12, 30, 45, tzinfo=_AbstractTz())
-        assert _iso8601_from_time(value) == "12:30:45"
-
-    def test_time_broken_tzinfo_preserves_microseconds(self) -> None:
-        from dqlitedbapi.types import _iso8601_from_time
-
-        value = datetime.time(12, 30, 45, 123456, tzinfo=_AbstractTz())
-        assert _iso8601_from_time(value) == "12:30:45.123456"
-
-    def test_time_broken_tzinfo_round_trips_through_fromisoformat(self) -> None:
-        from dqlitedbapi.types import _iso8601_from_time
-
-        original = datetime.time(12, 30, 45, 123456, tzinfo=_AbstractTz())
-        encoded = _iso8601_from_time(original)
-        decoded = datetime.time.fromisoformat(encoded)
-        assert decoded.tzinfo is None
-        assert decoded == datetime.time(12, 30, 45, 123456)
+        with pytest.raises(DataError, match="utcoffset"):
+            _iso8601_from_time(value)
 
 
 class TestConvertBindParamTime: