worker thread catching exceptions and putting them in queue

Discussion in 'Python' started by Paul Sijben, Mar 5, 2007.

  1. Paul Sijben

    Paul Sijben Guest

    All,

    in a worker thread setup that communicates via queues is it possible to
    catch exceptions raised by the worker executed, put them in an object
    and send them over the queue to another thread where the exception is
    raised in that scope?

    considering that an exception is an object I feel it ought to be
    possible, however I do not see how to go about it.

    does anyone have a pointer towards the solution?

    Paul
     
    Paul Sijben, Mar 5, 2007
    #1
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  2. Just raise the exception object found in the queue. Only make sure it _is_
    an exception, as you can raise everything. So in your queue-putting-code
    you might consider discriminating between the two cases, like this:


    while True:
    try:
    result = 1, work()
    except:
    result = 2, sys.exc_info()[1]
    queue.put(result)

    -------
    while True:
    kind, result = queue.get()
    if kind == 1:
    do(result)
    elif kind == 2:
    raise result


    Diez
     
    Diez B. Roggisch, Mar 5, 2007
    #2
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  3. Paul Sijben

    Stargaming Guest

    You're right, even exceptions are objects in Python. For further
    studies, read http://docs.python.org/lib/module-exceptions.html

    You can catch an exception like this:
    try:
    worker.do_some_work_that_may_raise_an_exception()
    except Exception, e:
    # the first argument is the type of error you want to handle
    # it is Exception here, the baseclass of all computation exceptions
    # the second argument is the variable (name) where you want to save
    # the specific exception raised to
    # it's 'e' here, a common shortcut for exception
    exception_handler.handle(e)
    # notice that you can pass e around as you like

    For further information on exceptions and how to handle them, read
    chapter 8 of the tutorial, especially starting from 8.3:
    http://docs.python.org/tut/node10.html#SECTION0010300000000000000000

    P.S. I don't know if what I told still applies to Python 3.0 -- a lot of
    changes are upcoming related to exception raising and handling.
     
    Stargaming, Mar 5, 2007
    #3
  4. Paul Sijben

    Paul Sijben Guest

    Stargaming & Diez,

    thanks very much!
     
    Paul Sijben, Mar 6, 2007
    #4
  5. Paul Sijben

    Aahz Guest

    One caution: because exceptions keep stack frames alive, you can have
    garbage collection problems. Make sure to del the exception when you're
    done with it.
     
    Aahz, Mar 8, 2007
    #5
  6. Note that it's the traceback who keeps stack frames alive, not the
    exception itself (at least on the current Python versions, might change in
    the future). If no fancy processing of the traceback is needed, maybe
    converting it to string (using the traceback module) before posting it to
    the queue is enough.
     
    Gabriel Genellina, Mar 8, 2007
    #6
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