My understanding is that internals of Queue
objects in python handle locking so that that you don't need to use locks externally just to call .put()
or .get()
. My question is am I covered by those same object internals if I want to display the contents of the queue by casting it as a list via list(queue) list(queue.queue)
? Or, more generally, is there any conceivable operation using python queues that require the use of locks external to the queue object to ensure thread safety?
Example:
from queue import Queue
queue = Queue()
list(queue.queue)
Phrased another way, will I ever have to use
with lock_object:
some_function(queue)
???
It's unclear what you mean by "Queue". The only two standard Queue
implementations I'm aware of cannot be iterated over, so "casting it as a list" just raises an exception:
>>> import queue
>>> q = queue.Queue()
>>> list(q)
Traceback (most recent call last):
...
TypeError: 'Queue' object is not iterable
>>> from multiprocessing import Queue
>>> q = Queue()
>>> list(q)
Traceback (most recent call last):
...
TypeError: 'Queue' object is not ierable
Whether you may ever need to do:
with lock_object:
some_function(queue)
can't be answered based on what you've said so far. If, for example, your higher-level logic relies on putting the queue into (in effect) "read only" mode for some time, then, sure. You'll need a lock to ensure mutual exclusion between the reading and writing sides for the duration.
.put()
and .get()
on their own are already thread- and process- (in the case of multiprocessing.Queue
) safe.
More than just that is not guaranteed by the docs, so can't be relied on even if it "appears to" work (which, if so, may be a reliable accident of the specific Python implementation you're using, or may be a fickle accident due to your simply not having yet bumped into a relevant race condition).
The question was edited to ask about list(queue.queue)
instead. That falls under the earlier "accident of the specific Python implementation you're using", in two respects:
Queue.queue
object has a queue
attribute. Python is a "consenting adults" language, and doesn't try to prevent you from using implementation details. But, if you do, you're on your own. The implementation may change at any time.list(deque)
is thread-safe today (CPython 3.12.5), but that's not documented either. I only know that it is because I stared at the C implementation code. It may not be thread-safe in 3.12.6. More generally, CPython is moving toward a "no GIL" mode of operation, in which this kind of thing becomes much more likely to suffer races.The bottom line doesn't change: .put()
and .get()
on their own are already thread- and process- (in the case of multiprocessing.Queue
) safe. Nothing more than that is guaranteed. Really! ;-) Nothing. If you need more than just that much to be reliable across implementations and releases, you'll need to supply your own locks.