Use get_nowait() in python without raising empty exception

Question

I have an inter-process queue that is usually empty and only once in a while does something appear in it. In one of my threads I want to define a while loop like this:

def mythread(queue1):

    while (queue1.get_nowait() != 1):
        #do stuff

This works great until the queue is empty which happens quickly in my case. When the queue is empty calling get_nowait() or get(False) raises the empty queue exception. Is there any way to check the queue without blocking AND without raising the empty exception?

If you don't want to block, then something has to happen when you call it on an empty queue. Catch the exception and handle it. — mata, Aug 26 '16 at 16:29

Jean-François Fabre · Accepted Answer · 2016-08-26T16:01:16.587

8

use empty method

if queue1.empty():
   pass
else:
   # get items from the queue

Note: doc says "not reliable" (no kidding).

I suppose that's because it can return True (queue is empty) while a message is posted, so the operation is not as atomic as catching the exception. Well, followed by a get_nowait() call I suppose it doesn't matter unless some other thread can consume the queue as well, in which case a racing condition could occur and the queue could be empty when you try to read from it!

edited Aug 26 '16 at 16:01

answered Aug 26 '16 at 15:58

Jean-François Fabre

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good idea, it looks like I need another while loop on top of the one I want – willpower2727 Aug 26 '16 at 16:00
1

Using `pass` for nothing? So, why not `if not queue1.empty(): ...`? – slayer Apr 01 '19 at 01:41
it's a "positive" branch pattern, to materialize the fact that we do nothing when the queue is empty. But yes, you can use `not queue1.empty()` – Jean-François Fabre Apr 01 '19 at 05:36

Use get_nowait() in python without raising empty exception

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