How to Catch a Thread's Exception in the Caller Thread in Python?

Catch a Thread's Exception in the Caller Thread

Multithreaded programming, tasks executed concurrently, provides efficiency in handling complex operations. However, exceptions encountered in threads can become challenging to manage. In Python, the standard "try-except" block may not handle exceptions raised in a child thread. This article explores a technique to capture exceptions from a thread in the caller thread, empowering developers with the ability to handle them effectively.

Understanding the Issue

Consider the following scenario: a script performs file copying in a separate thread, with the intention of displaying progress indicators. However, when file copying fails, an exception is thrown within the thread. The code snippet below attempts to handle this exception:

try:
    threadClass = TheThread(param1, param2, etc.)
    threadClass.start()   ##### **Exception takes place here**
except:
    print "Caught an exception"

Unfortunately, this approach fails because the "start" method returns immediately, causing the exception to occur in the child thread's context. The exception stack trace remains isolated within that thread, making it inaccessible to the caller thread.

Resolving the Exception

To address this issue, one promising approach involves using message passing. By employing a message queue, exceptions can be communicated from the child thread to the caller thread. Here's an example implementation:

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()

Explanation:

• ExcThread: A custom thread class that places exception information in a message queue named bucket.
• main: The caller thread that continuously checks the queue for exceptions using a non-blocking get operation.
• exc_type, exc_obj, exc_trace: Contents of the retrieved exception that can be handled accordingly.

Conclusion

By employing message passing techniques, it becomes possible to capture exceptions from a child thread and handle them effectively in the caller thread. This empowers developers to address potential errors swiftly, leading to more robust and responsive multithreaded applications.

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