Rospy: Why does the loop not get interrupted?

Question

I am working on the software for an autonomous Raspberry-Pi based robot which should keep moving for 2 minutes unless an unintended key is fed to it as input. Upon receipt of the unintended key, the robot must stop.

I appropriated the code from this tutorial and it looks like this (I removed some lines for brevity):

#Obstacle avoidance
HowNear= 15
timeLimit= 2
dt = rospy.Duration(secs=120)
reversetime= 0.5
turntime= 0.75
 
stop= False
  
#Set pins as output and input
GPIO.setup(pinTrigger, GPIO.OUT) #Trigger
GPIO.setup(pinEcho, GPIO.IN)    #Echo
   
# Set the GPIO Pin mode to be Output
GPIO.setup(pinMotorAForwards, GPIO.OUT)
GPIO.setup(pinMotorABackwards, GPIO.OUT)
GPIO.setup(pinMotorBForwards, GPIO.OUT)
GPIO.setup(pinMotorBBackwards, GPIO.OUT)
 
# Set the GPIO to software PWM at 'Frequency' Hertz
pwmMotorAForwards = GPIO.PWM(pinMotorAForwards, Frequency)
pwmMotorABackwards = GPIO.PWM(pinMotorABackwards, Frequency)
pwmMotorBForwards = GPIO.PWM(pinMotorBForwards, Frequency)
pwmMotorBBackwards = GPIO.PWM(pinMotorBBackwards, Frequency)
 
# Start the software PWM with a duty cycle of 0 (i.e. not moving)
pwmMotorAForwards.start(Stop)
pwmMotorABackwards.start(Stop)
pwmMotorBForwards.start(Stop)
pwmMotorBBackwards.start(Stop)
 
def myhook():
        print "shutting down"
 
# Turn all motors off
def stopmotors():
    pwmMotorAForwards.ChangeDutyCycle(Stop)
    pwmMotorABackwards.ChangeDutyCycle(Stop)
    pwmMotorBForwards.ChangeDutyCycle(Stop)
    pwmMotorBBackwards.ChangeDutyCycle(Stop)
 
 
# Turn both motors forwards
def forwards():
    pwmMotorAForwards.ChangeDutyCycle(DutyCycle)
    pwmMotorABackwards.ChangeDutyCycle(Stop)
    pwmMotorBForwards.ChangeDutyCycle(DutyCycle)
    pwmMotorBBackwards.ChangeDutyCycle(Stop)
  
def isnearobstacle(localhownear):
        distance= measure()
 
        print("IsNearObstacle: " + str(distance))
        if distance < localhownear:
                return True
        else:
                return False
 
def measure():
        GPIO.output(pinTrigger, True)
        time.sleep(0.00001)
        GPIO.output(pinTrigger, False)
        starttime= time.time()
        stoptime= starttime
 
        while GPIO.input(pinEcho)== 0:
                starttime= time.time()
                stoptime= starttime
 
 
        while GPIO.input(pinEcho)== 1:
                stoptime= time.time()
                if stoptime - starttime >= 0.04:
                        print("Too close/too far!")
                        stoptime= starttime
                        break
 
        elapsedtime= stoptime - starttime
        distance= (elapsedtime * 34300)/2
 
        return distance
 
def avoidobstacle():
        print("Backwards")
        backwards()
        time.sleep(reversetime)
        stopmotors()
 
        print("Right")
        right()
        time.sleep(turntime)
        stopmotors()
 
 
def PathScanning():    
        GPIO.output(pinTrigger, False) 
        time.sleep(0.1)
    
        globalStartTime= rospy.Time.now()
        globalStopTime= globalStartTime
 
        try:
                while True:
                   while not rospy.is_shutdown():
                        if stop == True:
                                print(stop)
                                stopmotors()
                        elif (globalStopTime-globalStartTime) < dt:
                                globalStopTime= rospy.Time.now()
                                forwards()
                                time.sleep(0.1)
                                if isnearobstacle(HowNear):
                                    stopmotors()
                                    avoidobstacle()
        except KeyboardInterrupt:
               print("Key Press Detected. Stopping Motors")
                stopmotors()
 
# Message handler
def CommandCallback(commandMessage):
        command = commandMessage.data  
        if command == 'A':
                PathScanning()
        else:
                print('Unknown command, stopping instead')
                stop = True
                print(stop)
                stopmotors()
    
rospy.init_node('driver', anonymous=True)
 
rospy.Subscriber('command', String, CommandCallback)
 
rospy.spin()
print('Shutting down: stopping motors')
 
 
stopmotors()
 
GPIO.cleanup()

The problematic portion of the code is as follows:

   def PathScanning():    
            GPIO.output(pinTrigger, False) 
            time.sleep(0.1)
        
            globalStartTime= rospy.Time.now()
            globalStopTime= globalStartTime
     
            try:
                    while True:
                       while not rospy.is_shutdown():
                            if stop == True:
                                    print(stop)
                                    stopmotors()
                            elif (globalStopTime-globalStartTime) < dt:
                                    globalStopTime= rospy.Time.now()
                                    forwards()
                                    time.sleep(0.1)
                                    if isnearobstacle(HowNear):
                                        stopmotors()
                                        avoidobstacle()
            except KeyboardInterrupt:
                   print("Key Press Detected. Stopping Motors")
                    stopmotors()
     
    # Message handler
    def CommandCallback(commandMessage):
            command = commandMessage.data  
            if command == 'A':
                    PathScanning()
            else:
                    print('Unknown command, stopping instead')
                    stop = True
                    print(stop)
                    stopmotors()
        
    rospy.init_node('driver', anonymous=True)
     
    rospy.Subscriber('command', String, CommandCallback)

My goal is to stop the robot from moving when the user passes any string/character besides 'A' as input. However, that does not happen when the PathScanning() function is already running. When I type any string besides 'A', I see the print statements on the console but stopmotors() is not called and the robot keeps moving (if 'A' was pressed earlier). I believe this is because PathScanning() has a loop in it:

   def PathScanning():    
            GPIO.output(pinTrigger, False) 
            time.sleep(0.1)

            globalStartTime= rospy.Time.now()
            globalStopTime= globalStartTime

            try:
                    while True:
                       while not rospy.is_shutdown():
                            if stop == True:
                                    print(stop)
                                    stopmotors()
                            elif (globalStopTime-globalStartTime) < dt:
                                    globalStopTime= rospy.Time.now()
                                    forwards()
                                    time.sleep(0.1)
                                    if isnearobstacle(HowNear):
                                        stopmotors()
                                        avoidobstacle()
            except KeyboardInterrupt:
                   print("Key Press Detected. Stopping Motors")
                    stopmotors()

When I remove the loop from PathScanning(), the robot stops when I provide any input besides 'A'. However, my other requirement of making the robot run the PathScanning() function for exactly two minutes is not fulfilled.

How is it possible that when I give any input besides 'A', the print statements in the branch are appropriately recognized but the stopmotors() function is not called?

Answer 1

Look in CommandCallback, where you set "stop = True". That sets a local variable, not the global variable you intended. Declare the variable to be global by preceding the assignment with the declaration "global stop".

This contrasts with reading a global variable, which just works without declaring it (notably, in PathScanning). It might not be a classic Python gotcha, but it has snagged me enough times that I look for it. One ref is https://www.geeksforgeeks.org/global-keyword-in-python/