docs: def _format_code(source, preferred_quote):
      """Return source code with quotes unified."""
      if not source:
          return source
  
      modified_tokens = []
  
      sio = io.StringIO(source)
      for (token_type,
           token_string,
           start,
           end,
           line) in tokenize.generate_tokens(sio.readline):
  
          if token_type == tokenize.STRING:
              token_string = unify_quotes(token_string,
                                          preferred_quote=preferred_quote)
  
          modified_tokens.append(
              (token_type, token_string, start, end, line))
  
      return untokenize.untokenize(modified_tokens)