Raise unhandled exceptions in a thread in the main thread?

Question

There are some similar questions, but none supply the answer I require.

If I create threads via threading.Thread, which then throw exceptions which are unhandled, those threads are terminated. I wish to retain the default print out of the exception details with the stack trace, but bring down the whole process as well.

I've considered that it might be possible to catch all exceptions in the threads, and reraise them on the main thread object, or perhaps it's possible to manually perform the default exception handling, and then raise a SystemExit on the main thread.

What's the best way to go about this?

Answer 1

I wrote about Re-throwing exceptions in Python, including something very much like this as an example.

On your worker thread you do this:

try:
    self.result = self.do_something_dangerous()
except Exception, e:
    import sys
    self.exc_info = sys.exc_info()

and on your main thread you do this:

if self.exc_info:
    raise self.exc_info[1], None, self.exc_info[2]
return self.result

The exception will appear in the main thread just as if it had been raised in the worker thread.

Answer 2

The only exception a secondary thread can reliably raise in the main thread is KeyboardInterrupt: the way the secondary thread does it is by calling the function thread.interrupt_main(). There is no way to associate extra info (about the reason for the exception) with the exception object that gets raised -- the latter's always just a plain KeyboardInterrupt. So, you need to stash that information away somewhere else, e.g. on a dedicated instance of Queue.Queue -- that info might include the results the secondary thread can get via sys.exc_info(), and anything else you find useful of course.

The main thread will need to recover that extra info (and take into account that the queue will be empty if the keyboard interrupt is actually due to the user hitting control-C or the like, so, use get_nowait and be ready to deal with a Queue.Empty exception, for example), format it however you desire, and terminate (if all secondary threads are daemons, the whole process terminates when the main thread terminates).