Fix mypy and add better comments.

src/palace/manager/scripts.py

@@ -4,15 +4,15 @@
 import os
 import sys
 import time
-from collections.abc import Sequence
+from collections.abc import Callable, Sequence
 from datetime import timedelta
 from pathlib import Path
 from typing import Any
 
 from alembic import command, config
 from alembic.util import CommandError
 from sqlalchemy import inspect, select
-from sqlalchemy.engine import Connection
+from sqlalchemy.engine import Connection, Engine
 from sqlalchemy.exc import NoResultFound
 from sqlalchemy.orm import Session
 
@@ -489,6 +489,7 @@ class InstanceInitializationScript:
     def __init__(
         self,
         config_file: Path | None = None,
+        engine_factory: Callable[[], Engine] = SessionManager.engine,
     ) -> None:
         self._log: logging.Logger | None = None
         self._container = container_instance()
@@ -497,6 +498,8 @@ def __init__(
         self._container.init_resources()
         self._config_file = config_file
 
+        self._engine_factory = engine_factory
+
     @property
     def log(self) -> logging.Logger:
         if self._log is None:
@@ -571,7 +574,7 @@ def run(self) -> None:
         instance of the script is running at a time. This prevents multiple
         instances from trying to initialize the database at the same time.
         """
-        engine = SessionManager.engine()
+        engine = self._engine_factory()
         with engine.begin() as connection:
             with pg_advisory_lock(connection, LOCK_ID_DB_INIT):
                 self.initialize(connection)