docs: def fost_hmac_url_signature(
      key, secret, host, path, query_string, expires):
      """
          Return a signature that corresponds to the signed URL.
      """
      if query_string:
          document = '%s%s?%s\n%s' % (host, path, query_string, expires)
      else:
          document = '%s%s\n%s' % (host, path, expires)
      signature = sha1_hmac(secret, document)
      return signature