How to active ControlEvent in ControlP5 - python Mode?

Processing.py
6 
"""
 ControlP5::addListener() (v1.1)
 GoToLoop (2018-Jun-20)

 https://Discourse.Processing.org/t/
 how-to-active-controlevent-in-controlp5-python-mode/1113/6
"""

add_library('controlP5')

bg = 0200

def setup():
    size(400, 300)

    global slider
    slider = ControlP5(this).\
             addSlider('slider').\
             setSize(width>>1, 20).\
             setPosition(width>>2, height>>1).\
             setRange(0, 0xff).\
             setValue(bg).\
             addListener(sliderListener)


def draw(): background(bg)


def sliderListener(evt):
    global bg
    bg = int(evt.getValue())