How to Catch Child Thread Exceptions in a Parent Thread in Python?

Handling Exceptions in Multithreaded Python Applications

In Python, multithreaded programming allows you to execute tasks concurrently. However, exception handling in multithreaded contexts can be challenging. This article addresses a specific issue: catching exceptions that occur in a child thread from the parent thread.

The problem arises because the child thread operates independently with its own context and stack. Exceptions thrown in the child thread are not immediately visible to the parent thread. Traditional try-except blocks in the parent thread, as shown below, will not work:

try:
    threadClass.start()   ##### **Exception takes place here**
except:
    print "Caught an exception"

To address this, we can utilize message passing. The child thread can send exception information to the parent thread using a shared data structure like a queue.

import sys
import threading
import queue

class ExcThread(threading.Thread):

    def __init__(self, bucket):
        threading.Thread.__init__(self)
        self.bucket = bucket

    def run(self):
        try:
            raise Exception('An error occured here.')
        except Exception:
            self.bucket.put(sys.exc_info())

def main():
    bucket = queue.Queue()
    thread_obj = ExcThread(bucket)
    thread_obj.start()

    while True:
        try:
            exc = bucket.get(block=False)
        except queue.Empty:
            pass
        else:
            exc_type, exc_obj, exc_trace = exc
            # deal with the exception
            print exc_type, exc_obj
            print exc_trace

        thread_obj.join(0.1)
        if thread_obj.isAlive():
            continue
        else:
            break

if __name__ == '__main__':
    main()

In this example:

1. A queue bucket is created to share exception information.
2. A ExcThread is initialized with a reference to the bucket.
3. The main thread continuously checks for exceptions in the queue.
4. If an exception is found, the main thread can handle it appropriately.

Using message passing allows for efficient communication of exceptions between threads.

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