code: def test_prebuild_index_returns_error(self, mock_popen):
      # See https://stackoverflow.com/a/36501078/866026
      mock_popen.return_value = mock.Mock()
      mock_popen_obj = mock_popen.return_value
      mock_popen_obj.communicate.return_value = ("", "Some Error")
      mock_popen_obj.returncode = 0
  
      index = search_index.SearchIndex(prebuild_index=True)
      expected = {
          "docs": [],
          "config": {"prebuild_index": True},
      }
      with self.assertLogs("mkdocs") as cm:
          result = json.loads(index.generate_search_index())
      self.assertEqual(
          "\n".join(cm.output),
          "WARNING:mkdocs.contrib.search.search_index:Failed to pre-build search index. Error: Some Error",
      )
  
      self.assertEqual(mock_popen.call_count, 1)
      self.assertEqual(mock_popen_obj.communicate.call_count, 1)
      self.assertEqual(result, expected)
  
imports: import json
  import unittest
  from unittest import mock
  from mkdocs.config.config_options import ValidationError
  from mkdocs.contrib import search
  from mkdocs.contrib.search import search_index
  from mkdocs.structure.files import File
  from mkdocs.structure.pages import Page
  from mkdocs.structure.toc import get_toc
  from mkdocs.tests.base import dedent, get_markdown_toc, load_config
label_desc: Use image node_id 10 for calling the SearchIndexTests obj's underlying member method code with example usage: obj.test_prebuild_index_returns_error(mock_popen) without return types