[Tutor] A question about None

[Mark Lawrence]

On 22/08/2021 08:25, Phil wrote:
> The distance sensor outputs "None" if the distance is greater than can 
> be measured and "None", of course cannot be compared. So I came up with 
> the following which is not quite correct:
> 
> while True

Tut tut not your actual code, while True: :)

>      if distance_sensor.get_distance_cm() is not None:
>          if distance_sensor.get_distance_cm() > 20:
>              motor_pair.set_default_speed(20)
>              motor_pair.start()
>          else:
>              continue
> 
>      elif distance_sensor.get_distance_cm() is not None:
>          if distance_sensor.get_distance_cm() < 20:
>              motor_pair.set_default_speed(-20)
>              motor_pair.start()
>          else:
>              continue
> 
> The idea is that if "None" returned from the sensor then nothing will 
> happen and the loop will continue until "None" is not returned. Can 
> anyone see the error of my ways?
> 

I think that you're over complicating it.  With your code the subsequent 
calls to get_distance_cm could return None so the comparison would fail 
anyway.  How about:-

while True
:
     distance = distance_sensor.get_distance_cm()
     if distance is not None:

         if distance > 20:

             speed = 20
         else:
             speed = -20
         motor_pair.set_default_speed(speed)

         motor_pair.start()

         break

There are other ways to structure the loop using flags but IMHO it's six 
of one, half a dozen of the other.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence