code: def null_db_instances():
      """Nullify singleton instance so that we can assure independent instantiation tests."""
      storage_factory.instance = None
      database_factory.instance = None
  
imports: import os
  import shutil
  import subprocess
  import pytest
  from pymongo import MongoClient
  import orion.core.cli
  import orion.core.io.experiment_builder
  from orion.client import create_experiment
  from orion.core.io.database import database_factory
  from orion.storage.base import storage_factory
label_desc: Use image node_id 14 for calling a global function with example usage: null_db_instances() without return types