code: def check_async(async_, kwargs, default):
      
      if async_ is not None:
          return async_
      elif 'async' in kwargs:
          warnings.warn(
              '"async" attribute is deprecated. Use "async_" instead.',
              DeprecationWarning,
          )
          return kwargs.pop('async')
      else:
          return default
docs: Return a value of 'async' in kwargs or default when async_ is None.
  
      This helper function exists for backward compatibility (See #274).
      It shows a warning message when 'async' in kwargs is used to note users.