Why can I not catch a Queue.Empty exception from a multiprocessing Queue?

Question

I’m trying to catch a Queue.Empty exception that is raised if a multiprocessing.Queue is empty. The following does not work:

import multiprocessing
f = multiprocessing.Queue()
try:
      f.get(True,0.1)
except Queue.Empty:
      print 'foo'

This gives me a name error: NameError: name ‘Queue’ is not defined

replacing Queue.Empty with multiprocessing.Queue.Empty does not help either. In this case it gives me a “AttributeError: ‘function’ object has no attribute ‘Empty'” exception.

Asked By: Alexander

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Source

Answer 1

The Empty exception you’re looking for isn’t available directly in the multiprocessing module, because multiprocessing borrows it from the queue module (which used to be named Queue in Python 2). To make your code work, just do an import queue at the top:

Try this:

import multiprocessing
import queue # or Queue in Python 2

f = multiprocessing.Queue()
try:
    f.get(True,0.1)
except queue.Empty: # queue here refers to the module, not a class
    print('foo')

Answered By: Blckknght

Answer 2

Blckknght’s answer from back in 2012 is still correct, however using Python 3.7.1 I discovered that you have to use queue.Empty as the name of the exception to catch (Note the lowercase ‘q’ in ‘queue’.)

So, to recap:

import queue

# Create a queue
queuevarname = queue.Queue(5) # size of queue is unimportant

while some_condition_is_true:
    try:
        # attempt to read queue in a way that the exception could be thrown
        queuedObject = queuevarname.get(False)

        ...
    except queue.Empty:
        # Do whatever you want here, e.g. pass so
        # your loop can continue, or exit the program, or...

Answered By: ggariepy

Why can I not catch a Queue.Empty exception from a multiprocessing Queue?

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