code: def test_fs():
      """Test the mock filesystem."""
      with mock_filesystem():
          with open("somefile", "w") as fd:
              fd.write(b"bla")
          with open("someotherfile", "w") as fd:
              fd.write(b"bli")
          with open("somefile", "r") as fd:
              assert fd.read() == b"bla"
          with open("someotherfile", "r") as fd:
              assert fd.read() == b"bli"
          assert os.path.isfile("somefile")
          assert not os.path.isfile("missingfile")
  
imports: from __future__ import annotations
  import contextlib
  import io
  import os
  import sys
  import tarfile
  import unittest
  from collections import defaultdict
  from unittest import mock
  import pytest
label_desc: Use image node_id 4 for calling a global function with example usage: test_fs() without return types