[Tutor] A question about None

[dn]

On 22/08/2021 20.25, dn via Tutor wrote:
> On 22/08/2021 19.25, 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
>>     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?
> 
> 
> What if the distance is 20(cm)?
while True
    measure = distance_sensor.get_distance_cm()
    if measure is not None:
        if measure > 20:
            motor_pair.set_default_speed(20)
        else:
            motor_pair.set_default_speed(-20)
        motor_pair.start()
> 
> while True
>     measure = distance_sensor.get_distance_cm()
>     if measure is not None:
>         if measure > 20:
>             motor_pair.set_default_speed(20)
>         else:
>             motor_pair.set_default_speed(-20)
>         motor_pair.start()
> 
> Assuming:
> - correct understanding of >, <, or =
> - start() applies in either case and as long as the sensor returned a value
> - consider adding a sleep() to the loop, depending upon the necessary
> frequency of sensing

Further thoughts:-

1 improve documentation, and
2 remove 'magic values' which are embedded/hidden within the code

forwards = 1
backwards = -1
speed = 20
significant_distance = 20
...

while True
    measure = distance_sensor.get_distance_cm()
    if measure is not None:
        if measure > significant_distance:
            direction = forwards
        else:
            direction = backwards
        motor_pair.set_default_speed( speed * forwards )
        motor_pair.start()

Am wondering why 'speed' is not an argument to start()?

The if-condition needs closer analysis, and could be compressed into a
single-line (if meaning remains clear)

Also, could replace the outer if-construct with a try-except.
Ultimately, this might give you the opportunity to count the number of
consecutive zero-responses from the sensor, in order to take appropriate
action.

-- 
Regards,
=dn