code: def _generate_sas_token(uri, policy, key, expiry=None):
      
      from base64 import b64encode, b64decode
      from hashlib import sha256
      from hmac import HMAC
      if not expiry:
          expiry = time.time() + 3600  # Default to 1 hour.
      encoded_uri = quote_plus(uri)
      ttl = int(expiry)
      sign_key = '%s\n%d' % (encoded_uri, ttl)
      signature = b64encode(HMAC(b64decode(key), sign_key.encode('utf-8'), sha256).digest())
      result = {
          'sr': uri,
          'sig': signature,
          'se': str(ttl)}
      if policy:
          result['skn'] = policy
      return 'SharedAccessSignature ' + urlencode(result)
docs: Create a shared access signiture token as a string literal.
      :returns: SAS token as string literal.
      :rtype: str