Import _DescriptionTuple from the dbapi layer in the async adapter

The async adapter re-declared ``_DescriptionTuple`` alongside the
dbapi layer with a comment explaining the defensive duplication.
But dbapi is a hard dependency of sqlalchemy-dqlite, so there is
no import boundary to preserve — and the two aliases can only
drift by silent modification of one side. Import the tuple shape
from the dbapi layer instead and keep the outer
``Sequence[...]`` wrapper so the adapter's permissive passthrough
contract (accepting list or tuple outputs) is unchanged.

A new pinning test asserts the two aliases are identity-equal so a
future refactor reintroducing a local copy would fail loudly.

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

Author: antoineleclair

src/sqlalchemydqlite/aio.py

@@ -19,6 +19,7 @@
     OperationalError,
     ProgrammingError,
 )
+from dqlitedbapi.types import _DescriptionTuple
 from sqlalchemydqlite.base import DqliteDialect
 
 logger = logging.getLogger(__name__)
@@ -28,19 +29,14 @@
 
 __all__ = ["AsyncAdaptedConnection", "AsyncAdaptedCursor", "DqliteDialect_aio"]
 
-# Description tuple shape per PEP 249 (name, type_code, display_size,
-# internal_size, precision, scale, null_ok). dqlite populates only
-# ``name`` and ``type_code`` — the other five are always ``None``.
-# Re-declared here (not imported) to keep the sqlalchemy-dqlite
-# runtime contract explicit to type-checkers even if the dbapi layer
-# later exposes a named alias.
 # PEP 249 specifies ``cursor.description`` as a sequence of sequences —
 # a ``list[tuple]`` is the canonical shape but a strict type alias of
 # ``list`` would reject a dbapi cursor that returns a tuple-of-tuples
-# (which sqlalchemy's own aiosqlite adapter accepts). Keep the alias
-# permissive so the adapter passes through whatever the underlying
-# cursor returns without copying.
-_DescriptionTuple = tuple[str, int | None, None, None, None, None, None]
+# (which sqlalchemy's own aiosqlite adapter accepts). Widen the outer
+# alias to ``Sequence`` so the adapter passes through whatever the
+# underlying cursor returns without copying. The inner 7-tuple shape is
+# imported from the dbapi layer (single source of truth) so a future
+# column (real display_size, etc.) propagates here automatically.
 _Description = Sequence[_DescriptionTuple] | None
 
 

tests/test_description_alias_shared.py

@@ -0,0 +1,16 @@
+"""Pin the description-tuple alias to the dbapi-layer single source
+of truth. If a future refactor re-declares ``_DescriptionTuple``
+locally it must not drift from the dbapi shape; importing ensures
+drift can only happen by deliberate coordinated change to both
+modules.
+"""
+
+from __future__ import annotations
+
+from dqlitedbapi.types import _DescriptionTuple as _DbapiDescriptionTuple
+from sqlalchemydqlite.aio import _DescriptionTuple as _AdapterDescriptionTuple
+
+
+def test_description_tuple_alias_is_shared() -> None:
+    """The adapter imports the dbapi-layer alias — ``is`` must hold."""
+    assert _AdapterDescriptionTuple is _DbapiDescriptionTuple