Exception-Raising Thread

exceptionthread.py

import Queue
import threading
import sys

__author__ = 'psobot'

class ExceptionThread(threading.Thread):
    def __init__(self, *args, **kwargs):
        threading.Thread.__init__(self, *args, **kwargs)
        self.__status_queue = Queue.Queue()

    def run(self, *args, **kwargs):
        try:
            threading.Thread.run(self)
        except Exception:
            self.__status_queue.put(sys.exc_info())
        self.__status_queue.put(None)

    def join(self, num=None):
        if not self.__status_queue:
            return
        try:
            exc_info = self.__status_queue.get(True, num)
            if exc_info:
                raise exc_info[1], None, exc_info[2]
        except Queue.Empty:
            return
        self.__status_queue = None

Base class for a thread that tracks its own exceptions and raises them when joined by the main thread? Clever. I've always been tempted to just output them to stderr as they happen.

Useful when an app has exception-based flow control, as is somewhat common in Python. My situation was one where I had to run a couple parallel tasks, but any one of them could fail (and log error output gracefully), making the entire operation invalid. I can then also do a .join(0) to check if a thread has failed yet, before continuing on with the other threads' execution.

Fixed this to properly raise exceptions with their original context. Before, all exceptions raised here would print:

File "/var/www/helpers/exceptionthread.py", line 24, in join
    raise thread_exception

Now, they show the original raiser of the exception:

File "exceptiontest.py", line 4, in fun
    raise Exception("Hey, look at me, on line 4!")