Use nullpool, lazy create engine, close current connection

dlt/destinations/impl/sqlalchemy/db_api_client.py

@@ -95,10 +95,9 @@ class DbApiProps:
 
 class SqlalchemyClient(SqlClientBase[Connection]):
     external_engine: bool = False
-    dialect: sa.engine.interfaces.Dialect
-    dialect_name: str
     dbapi = DbApiProps  # type: ignore[assignment]
     migrations: Optional[MigrationMaker] = None  # lazy init as needed
+    _engine: Optional[sa.engine.Engine] = None
 
     def __init__(
         self,
@@ -111,20 +110,36 @@ def __init__(
         super().__init__(credentials.database, dataset_name, staging_dataset_name, capabilities)
         self.credentials = credentials
 
+        self.engine_args = engine_args or {}
+
         if credentials.engine:
-            self.engine = credentials.engine
+            self._engine = credentials.engine
             self.external_engine = True
         else:
-            self.engine = sa.create_engine(
-                credentials.to_url().render_as_string(hide_password=False),
-                **(engine_args or {}),
-            )
+            # Default to nullpool because we don't use connection pooling
+            self.engine_args.setdefault("poolclass", sa.pool.NullPool)
 
         self._current_connection: Optional[Connection] = None
         self._current_transaction: Optional[SqlaTransactionWrapper] = None
         self.metadata = sa.MetaData()
-        self.dialect = self.engine.dialect
-        self.dialect_name = self.dialect.name
+
+    @property
+    def engine(self) -> sa.engine.Engine:
+        # Create engine lazily
+        if self._engine is not None:
+            return self._engine
+        self._engine = sa.create_engine(
+            self.credentials.to_url().render_as_string(hide_password=False), **self.engine_args
+        )
+        return self._engine
+
+    @property
+    def dialect(self) -> sa.engine.interfaces.Dialect:
+        return self.engine.dialect
+
+    @property
+    def dialect_name(self) -> str:
+        return self.dialect.name
 
     def open_connection(self) -> Connection:
         if self._current_connection is None:
@@ -136,6 +151,8 @@ def open_connection(self) -> Connection:
     def close_connection(self) -> None:
         if not self.external_engine:
             try:
+                if self._current_connection is not None:
+                    self._current_connection.close()
                 self.engine.dispose()
             finally:
                 self._current_connection = None