fix: make DatabaseSessionService visible to API documentation (#4331)

src/google/adk/sessions/__init__.py

@@ -11,12 +11,53 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
+from typing_extensions import override
+
 from .base_session_service import BaseSessionService
 from .in_memory_session_service import InMemorySessionService
 from .session import Session
 from .state import State
 from .vertex_ai_session_service import VertexAiSessionService
 
+try:
+  from .database_session_service import DatabaseSessionService
+except ImportError:
+  # This handles the case where optional dependencies (like sqlalchemy)
+  # are not installed. A placeholder class ensures the symbol is always
+  # available for documentation tools and static analysis.
+  class DatabaseSessionService(BaseSessionService):
+    """Placeholder for DatabaseSessionService when dependencies are not installed."""
+
+    _ERROR_MESSAGE = (
+        'DatabaseSessionService requires sqlalchemy>=2.0, please ensure it is'
+        ' installed correctly.'
+    )
+
+    def __init__(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def create_session(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def get_session(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def list_sessions(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def delete_session(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+    @override
+    async def append_event(self, *args, **kwargs):
+      raise ImportError(self._ERROR_MESSAGE)
+
+
 __all__ = [
     'BaseSessionService',
     'DatabaseSessionService',
@@ -25,17 +66,3 @@
     'State',
     'VertexAiSessionService',
 ]
-
-
-def __getattr__(name: str):
-  if name == 'DatabaseSessionService':
-    try:
-      from .database_session_service import DatabaseSessionService
-
-      return DatabaseSessionService
-    except ImportError as e:
-      raise ImportError(
-          'DatabaseSessionService requires sqlalchemy>=2.0, please ensure it is'
-          ' installed correctly.'
-      ) from e
-  raise AttributeError(f'module {__name__!r} has no attribute {name!r}')