How to get the items in Queue without removing the items?

Question

get() removes and returns an item from Queue in Python.

import queue

q = queue.Queue() # Here

q.put("Apple")
q.put("Orange")
q.put("Banana")

print(q.get())
print(q.get())
print(q.get())

Output:

Apple
Orange
Banana

Now, I want to get the items in Queue without removing the items.

Is it possible to do this?

Kendre

It sounds like you should be using a collections.deque instead, are you using threads? – Appolonia

no I am not using threads – Kendre

Answer 1

The Queue module implements multi-producer, multi-consumer queues. It is especially useful in threaded programming when information must be exchanged safely between multiple threads.

As you can see, the Queue module was created specifically for use with threads, providing only FIFO, LIFO and priority queues, none of which provide this functionality. However by examining the source code of the Queue module, you can see that it simply uses a collections.deque (double ended queue) which can easily accomplish your task. You may index the first item ([0]) and .popleft() in constant time.

Appolonia

To avoid misunderstandings, perhaps you should have mentioned (for other readers) something about Queue's having a self.mutex = _threading.Lock(), but not deques, so in a multithreaded environment using the latter instead of the former is not equivalent—which is not the case here since the OP mentioned that threads were not being used a comment. – Latticework

Answer 2

queue_object.queue will return copy of your queue in a deque object which you can then use the slices of. It is of course, not syncronized with the original queue, but will allow you to peek at the queue at the time of the copy.

There's a good rationalization for why you wouldn't want to do this explained in detail in this thread comp.lang.python - Queue peek?. But if you're just trying to understand how Queue works, this is one simple way.

import Queue
q = Queue.Queue()
q.push(1)
q.put('foo')
q.put('bar')
d = q.queue
print(d)
deque(['foo', 'bar'])
print(d[0])
'foo'

Turnery

And you -1 one me because I answered his question correctly? .. That really is rude. You're not taking into account that he may be working with existing code that requires Queue mutexes and he's just trying to understand how it works - to observe the queue at all. To dismiss the question (which is a reasonable newbie question), or -1 my correct answer is counter to the spirit of stackoverflow and to trying to help people. – Turnery

I -1d because you didn't answer it correctly, you didn't mention how to remove it as well, also it is the wrong task for the job (OP doesn't require Queue mutexes as mentioned in the comments) – Appolonia

Question was: 'Is it possible to examine items in a Queue in python without calling get()?'. Yes. It is. And I explained how. For the OP it's great he can switch to deque, other people with established code bases may not be able to. – Turnery

" .I need to check if the item at the head of queue satisfies some condition and if it does,remove it from the queue. .Similarly I need to check if any other item in queue satisfy the similar condition and remove it." – Appolonia

While the .queue attribute of Queue objects doesn't have a leading _ indicating it's private, it's undocumented, so I would be reluctant to use it. I won't downvote your answer, but I don't think it's good advice. – Latticework

@Appolonia You're both right. stackoverflow has a serious problem where it's unclear whether answers should be pedantic ("your question implies you're doing something wrong") or pragmatic ("here's how you do that unsafe thing you asked to do"). The correct advice is: ALWAYS DO BOTH. 1. Give the unsafe method AND 2. Explain why it shouldn't be used. When answering questions you have no idea why someone asked them. – Transcribe

Answer 3

It is NOT safe to simply access the underlying queue.

The safe way to do it is to extend the Queue class. If you return the underlying dequeue object, you will NOT be getting a copy, you get the live object.

The result of this is that it can change while you are iterating it - which will result in an exception if another thread inserts into the queue during your iteration.

Knowing that python uses the GIL, you can safely use list(q.queue), because list() will never cause a context switch.

It's better to use the same lock the get() function uses, and not make assumptions about the GIL:

import queue
    
class SnapshotQueue(queue.Queue):
    def snapshot(self):
        with self.mutex:
            return list(self.queue)

That class can be used safely instead of a regular queue, and it will return a snapshot of the queue state... within a mutex and without causing issues in the underlying queue operation.

Transcribe

nit: you're missing your colon on the with line. But this is a good answer. – Bravissimo

Answer 4

You can get the items in a queue without removing the items as shown below:

import queue

q = queue.Queue()

q.put("Apple")
q.put("Orange")
q.put("Banana")

print(q.queue[0]) # Here
print(q.queue[1]) # Here
print(q.queue[2]) # Here

print(q.queue) # Here

Output:

Apple
Orange
Banana
deque(['Apple', 'Orange', 'Banana'])

And, you can also change the items in a queue as shown below:

import queue

q = queue.Queue()

q.put("Apple")
q.put("Orange")
q.put("Banana")

q.queue[0] = "Strawberry" # Here
q.queue[1] = "Lemon"      # Here
q.queue[2] = "kiwi"       # Here

print(q.queue[0])
print(q.queue[1])
print(q.queue[2])

print(q.queue)

Output:

Strawberry
Lemon
kiwi
deque(['Strawberry', 'Lemon', 'kiwi'])

But, you cannot add items to a queue without put() as shown below:

import queue

q = queue.Queue()

q.queue[0] = "Apple"  # Cannot add
q.queue[1] = "Orange" # Cannot add
q.queue[2] = "Banana" # Cannot add

print(q.queue[0])
print(q.queue[1])
print(q.queue[2])

print(q.queue)

Then, the error below occurs:

IndexError: deque index out of range

Gland

Answer 5

I found this question, because I needed a way to access top element in a PriorityQueue. I couldn't find a way to do that, so I switched to heapq instead. Although it's worth mentioning that heapq is not thread safe.

Azpurua