code: def get_sig_string(req, cano_req, scope):
          
          amz_date = req.headers['x-amz-date']
          hsh = hashlib.sha256(cano_req.encode())
          sig_items = ['AWS4-HMAC-SHA256', amz_date, scope, hsh.hexdigest()]
          sig_string = '\n'.join(sig_items)
          return sig_string
docs: Generate the AWS4 auth string to sign for the request.
  
          req      -- Requests PreparedRequest object. This should already
                      include an x-amz-date header.
          cano_req -- The Canonical Request, as returned by
                      get_canonical_request()