尝试在无限while循环内阻止导致线程退出

Question

I'm working on a Python program that uses a thread to infinitely read from a serial connection and handle exceptions based on input.

In essence: Within a thread, I want to do-something forever, raise an exception if something happens, return from exception and continue to do-something forever while handling any future exceptions.

I've created a test program to emulate this behavior using time.sleep() in order to isolate my problem.

import threading
import time

class TimeError(Exception):
    def __init__(self, mesg):
        self.mesg=mesg

class myObject(object):

    def __init__(self):
        self.threadRunning=False

    def my_thread(self):
        threadCount=0

        try:
            while True:
                time.sleep(1)

                print "Child: {}".format(threadCount)
                threadCount = threadCount + 1
                if threadCount>=5:
                    raise TimeError("5 seconds elapsed!")

        except TimeError as e:
            print (e.mesg)

        print "Exiting thread!"
        self.threadRunning=False

    def my_parent(self): #start thread and wait
        childThread = threading.Thread(target=self.my_thread)
        childThread.daemon = True

        childThread.start()
        self.threadRunning = True

        parentCount=0
        while (self.threadRunning == True):
            print "Parent: {}".format(parentCount)
            parentCount = parentCount + 1
            time.sleep(1)

        print "Program complete."


fooObject = myObject()
fooObject.my_parent()

The output I received is

Parent: 0
Child: 0
Parent: 1
Child: 1
 Parent: 2
Child: 2
 Parent: 3
Child: 3
 Parent: 4
Child: 4
5 seconds elapsed!
Exiting thread!
 Parent: 5
Program complete.

I'm fairly new to Python and using exceptions (my background is C). I understand that exceptions are used to debug and recover from anomalies in normal execution flow. I've read that it's unwise to throw your own Exceptions , but for my final application, I must.

So, how do I get back to the While True infinite loop after handling an exception in the thread? I want to keep incrementing threadCount and handling the exception, not exit the thread.

Answer 1

By boiling down my code in order to ask this question I've found the solution.

In short, the while True loop should be surrounding the try/except block, not the other way around. This is because...

If an exception occurs during execution of the try clause, the rest of the >clause is skipped. Then if its type matches the exception named after the >except keyword, the except clause is executed, and then execution continues >after the try statement.

https://docs.python.org/2/tutorial/errors.html#handling-exceptions

My assumption was that an Exception behaved like an interrupt in that it went off to execute the handling code, then would return to the point in execution just after the Exception was raised. This isn't C...

Fixed code:

import threading
import time

class TimeError(Exception):
    def __init__(self, mesg):
        self.mesg=mesg

class myObject(object):

    def __init__(self):
        self.threadRunning=False

    def my_thread(self):
        threadCount=0


        while True:
            try:
                time.sleep(1)

                print "Child: {}".format(threadCount)
                threadCount = threadCount + 1
                if threadCount>=5:
                    raise TimeError("5 seconds elapsed!")

            except TimeError as e:
                print (e.mesg)

        print "Exiting thread!"
        self.threadRunning=False

    def my_parent(self): #start thread and wait
        childThread = threading.Thread(target=self.my_thread)
        childThread.daemon = True

        childThread.start()
        self.threadRunning = True

        parentCount=0
        while (self.threadRunning == True):
            print "Parent: {}".format(parentCount)
            parentCount = parentCount + 1
            time.sleep(1)

        print "Program complete."


fooObject = myObject()
fooObject.my_parent()

Intended output:

Parent: 0
Child: 0
Parent: 1
Child: 1
Parent: 2
Child: 2
Parent: 3
Child: 3
Parent: 4
Child: 4
 5 seconds elapsed!
Parent: 5
Parent: 6
 Child: 5
5 seconds elapsed!
Child: 6
 5 seconds elapsed!
Parent: 7
Parent: 8
 Child: 7
5 seconds elapsed!
Child: 8
 5 seconds elapsed!
...indefinately