code: def url_to_filename(url: str, etag: str = None) -> str:
      
      url_bytes = url.encode('utf-8')
      url_hash = sha256(url_bytes)
      filename = url_hash.hexdigest()
  
      if etag:
          etag_bytes = etag.encode('utf-8')
          etag_hash = sha256(etag_bytes)
          filename += '.' + etag_hash.hexdigest()
  
      return filename
docs: Convert `url` into a hashed filename in a repeatable way.
      If `etag` is specified, append its hash to the url's, delimited
      by a period.