Exception handling while iterating over Python queues

Question

I've got a bunch of queues stored in the global globqueue array. I now want to iterate over all these queues and get all the items inside of them.

I'm having a few problems with getting the items from the queue, though. I'm using .get_nowait(), which will throw an "Empty" exception if there is nothing in the queue. I thought I could catch it like this:

                    for index, item in enumerate(globqueue):
                        print index, item

                        iterme = 1

                        while iterme:
                            try:
                                getiterme = item.get_nowait()
                                print getiterme
                            except ValueError:
                                iterme = 0
                                continue

But I'm still getting this error, and the rest of the code won't continue:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "server.py", line 213, in run
    getiterme = item.get_nowait()
  File "/usr/lib/python2.6/Queue.py", line 190, in get_nowait
    return self.get(False)
  File "/usr/lib/python2.6/Queue.py", line 165, in get
    raise Empty
Empty

I know I could check the size of the queue first with .qsize(), but I also read that isn't always so accurate so.. better to ask for forgiveness than permission, right?

Artiom Kozyrev · Accepted Answer · 2021-05-13 09:01:48Z

6

You are catching ValueError but the call raises Empty. Try changing your except handler to catch the Empty exception instead.

from _queue import Empty

if __name__ == '__main__':
    try:
        item = item.get_nowait()
        # do some work
    except Empty:
        pass # handle error

edited May 13, 2021 at 9:01

Artiom Kozyrev

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answered Dec 25, 2010 at 21:24

Mark Byers

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4 Comments

Jelle De Loecker Over a year ago

Oh, I didn't know the part behind 'except' was that important. Queue.Empty didn't work, though, but changing it to 'Exception' (which contains "all built-in, non-system-exiting exceptions") it did work. Thanks

user395760 Over a year ago

@skerit: except Exception: (or even worse, except:) is very dangerous. It even catched NameError - i.e. silly typos and other errors you want to learn about will be silenced. How did the proper solution "not work"?

Jelle De Loecker Over a year ago

Oh, I'm sorry. I typed "queue", without a capital. But the error message then said AttributeError: Queue instance has no attribute 'Empty' which made me think I did write it with a capital. Indeed, Queue.Empty works as it should.

Mark Byers Over a year ago

@ckerit: I mistyped it in an earlier version of my answer. Sorry about that.

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