code: def discover_by_file(self, start_filepath, top_level_directory=None):
          
          start_filepath = os.path.abspath(start_filepath)
          start_directory = os.path.dirname(start_filepath)
          if top_level_directory is None:
              top_level_directory = find_top_level_directory(
                  start_directory)
          logger.debug('Discovering tests in file: start_filepath=%r, '
                       'top_level_directory=', start_filepath,
                       top_level_directory)
  
          assert_start_importable(top_level_directory, start_directory)
  
          if top_level_directory not in sys.path:
              sys.path.insert(0, top_level_directory)
          tests = self._load_from_file(
              start_filepath, top_level_directory)
          return self._loader.create_suite(list(tests))
docs: Run test discovery on a single file.
  
          Parameters
          ----------
          start_filepath : str
              The module file in which to start test discovery.
          top_level_directory : str
              The path to the top-level directoy of the project.  This is
              the parent directory of the project'stop-level Python
              package.