Move motor to absolute position

How can I set the motor A to 20°, for example?

import mindstorms
import time

hub = mindstorms.Hub()

time.sleep(1)

motor_a = hub.port.A.motor
motor_a.run_to_position(20)

Because this code only moves the motor +20° relative to its position at start-up. If this is not yet possible, adding such a method would be great.