code: def test_mtime_ns_is_none_for_memory(app_client):
      memory_db = Database(app_client.ds, is_memory=True)
      assert memory_db.is_memory is True
      assert None is memory_db.mtime_ns
  
file_path: datasette/tests/test_internals_database.py
imports: from datasette.app import Datasette
  from datasette.database import Database, Results, MultipleValues
  from datasette.utils.sqlite import sqlite3
  from datasette.utils import Column
  from .fixtures import app_client, app_client_two_attached_databases_crossdb_enabled
  import pytest
  import time
  import uuid
label_desc: Use image node_id 4 for calling a global function with example usage: test_mtime_ns_is_none_for_memory(app_client) without return types