code: def request_body(request):
      
      client_id = request.post_param("client_id")
      if client_id is None:
          raise OAuthInvalidError(error="invalid_request",
                                  explanation="Missing client identifier")
  
      client_secret = request.post_param("client_secret")
      if client_secret is None:
          raise OAuthInvalidError(error="invalid_request",
                                  explanation="Missing client credentials")
  
      return client_id, client_secret
docs: Extracts the credentials of a client from the
      *application/x-www-form-urlencoded* body of a request.
  
      Expects the client_id to be the value of the ``client_id`` parameter and
      the client_secret to be the value of the ``client_secret`` parameter.
  
      :param request: The incoming request
      :type request: oauth2.web.Request
  
      :return: A tuple in the format of `(<CLIENT ID>, <CLIENT SECRET>)`
      :rtype: tuple