How to get elapsed time in if conditional statement in pyhton-CodePudding

I'm currently working on a Raspberry Pi-based project about a car speed detection prototype. I used 2 Infrared Proximity sensors as a trigger. prox1 is Sensor 1 to trigger start stopwatch. prox2 is sensor 2 to trigger stop the stopwatch. The given distance is 50 cm. When a car passes both of the sensors, Raspberry Pi has to measure the speed (distance / (stop_time - start_time). And if the speed is above 60 cm/s, the camera would capture the car image and save it in pi folder. I want the prox1 and prox2 to be a trigger to start and stop the stopwatch. My question is, I keep getting warnings saying Name 'start_time' can be undefined. What should I do to make this work? Or is there any other way to get elapsed time triggered by the sensor without declaring the variable in the if-condition statement?

import time
from pint import UnitRegistry
from picamera import PiCamera

ureg = UnitRegistry()

camera = PiCamera()
camera.resolution = (1280, 720)
camera.exposure_mode = 'sports'
camera.iso = 0
camera.shutter_speed = 7500

GPIO.setwarnings(False)

prox1 = 12
prox2 = 18
led = 16

GPIO.setmode(GPIO.BOARD)
GPIO.setup(prox1, GPIO.IN)
GPIO.setup(prox2, GPIO.IN)
GPIO.setup(led, GPIO.OUT)

distance = 50 * ureg.cm

try:
    while True:
        if (GPIO.input(prox1) == False):  # object in front of the sensor = false
            start_time = time.time()
            GPIO.output(led, GPIO.HIGH)
            print('Calculating Speed')
            time.sleep(0.3)
        else:
            print('No Car Detected')
            time.sleep(1)

        if (GPIO.input(prox2) == False):
            stop_time = time.time()
            GPIO.output(led, GPIO.LOW)                
            timer: float = stop_time - start_time
            print('Time = ', timer)
            time.sleep(0.3)
            speed: float = distance / timer
            print('Speed = ', speed)
            time.sleep(0.3)
        if speed > 60 and timer < 1 * ureg.second :
            try:
                camera.capture('test1.jpg')
                time.sleep(2)
                pass
            finally:
                camera.close()

except KeyboardInterrupt:
    GPIO.cleanup()

CodePudding user response：

The problem is that start_time doesn't exist unless and until GPIO.input(prox1) returns False. This may not be possible in reality but from the code analysis perspective it could happen. Just set start_time to zero (or some other arbitrary value) immediately inside the while True loop