code: def test_default_attributes(monkeypatch):
      """Test the default attributes when no environment variables are set."""
      # pretend mpi4py is available
      monkeypatch.setattr(
          lightning.fabric.plugins.environments.mpi,
          "_MPI4PY_AVAILABLE",
          True,
      )
      mpi4py_mock = MagicMock()
      with mock.patch.dict("sys.modules", {"mpi4py": mpi4py_mock}):
          env = MPIEnvironment()
  
      assert env._node_rank is None
      assert env._main_address is None
      assert env._main_port is None
      assert env.creates_processes_externally
  
file_path: pytorch-lightning/tests/tests_fabric/plugins/environments/test_mpi.py
imports: import logging
  import os
  from unittest import mock
  from unittest.mock import MagicMock
  import lightning.fabric.plugins.environments.mpi
  import pytest
  from lightning.fabric.plugins.environments import MPIEnvironment
label_desc: Use image node_id 3 for calling a global function with example usage: test_default_attributes(monkeypatch) without return types