message[i]=int(message[i])报错"str" object doesn't support item assignment

设置PWM频率为50HZ，一个周期就是20ms，用ch1.pulse_width_percent(5)即为5%的占空比，也就是1ms；那么如果用ch1.pulse_width()，相同的占空比需要设置括号内的数字为多少？

我的初始化代码如下

import time
from pyb import Pin, Timer

tim = Timer(4, freq=50)#Time4；50HZ
ch1 = tim.channel(1, Timer.PWM, pin=Pin("P7"), pulse_width_percent=0)
ch2 = tim.channel(2, Timer.PWM, pin=Pin("P8"), pulse_width=0)


while (True):