Can I get an item from a PriorityQueue without removing it yet?

Question

I want to get the next item in queue but I don’t want to dequeue it. Is it possible in Python’s queue.PriorityQueue? From the docs, I don’t see how can it be done

Asked By: Jiew Meng

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Source

Answer 1

Indexing the first element of the queue should work. If you’re using the heapq library, the document mentions:

The interesting property of a heap is that its smallest element is always the root, heap[0].

Answered By: Aayush Kumar

Answer 2

When you get item form the queue as per theory it will remove from the queue. You have to write your own function which will give you last element of PriorityQueue. You can create a peek function by inherit the priorityqueue.

Answered By: Nilesh

Answer 3

If a is a PriorityQueue object, You can use a.queue[0] to get the next item:

from queue import PriorityQueue

a = PriorityQueue()

a.put((10, "a"))
a.put((4, "b"))
a.put((3,"c"))

print(a.queue[0])
print(a.queue)
print(a.get())
print(a.queue)
print(a.get())
print(a.queue)

output is :

(3, 'c')
[(3, 'c'), (10, 'a'), (4, 'b')]
(3, 'c')
[(4, 'b'), (10, 'a')]
(4, 'b')
[(10, 'a')]

but be careful about multi thread access.

Answered By: HYRY

Answer 4

If you want next element in the PriorityQueue, in the order of the insertion of the elements, use:

for i in range(len(queue.queue)):
    print queue.queue[i]

this will not pop anything out.

If you want it in the priority order, use:

for i in range(len(queue.queue)):
    temp = queue.get()
    queue.put(temp)
    print temp

If you are using a tuple, instead of a single variable, replace temp by:

((temp1,temp2))

Answered By: Victor Juliet

Answer 5

Assuming your items stored in the PriorityQueue is a tuple (priority, value),

def peek(pq):
  return pq.queue[0][1]

Answered By: hamster ham

Answer 6

If q is the PeriorityQueue, then you can use:

for i in range(q.qsize()):
    print(q.queue[i])

Answered By: ahsan naeem

Answer 7

TL;DR — If you are are using the top item multiple times in a multithreaded environment, you should assign the item to a variable.

The above answers show how to access the elements of a PriorityQueue, however there is some danger when accessing objects from the inner queue in a multithreaded manner (as mentioned at the end of HYRY’s answer).

While holding onto the queue object would be even less threadsafe, because that queue object is "live" (see this answer), you can still run into issues when accessing the objects on the queue if you expect the object ordering to be static.

Here’s an example:

from queue import PriorityQueue          
import threading                         
import time                              
                                         
pq = PriorityQueue()                     
                                         
def queue_manip():                       
    i = 0                                
    while True:                          
        pq.put((i, i))                   
        i -= 1                           
        time.sleep(.1)                   
                                         
t1 = threading.Thread(target=queue_manip)
t1.start()                               
                                         
while True:                              
    print('FirstAccess:  ', pq.queue[0]) 
    time.sleep(.2)  # other processing   
    print('SecondAccess: ', pq.queue[0]) 
    time.sleep(.2)  # other processing   
    print()

which will print something like the below:

FirstAccess:   (0, 0)  
SecondAccess:  (-1, -1)
                       
FirstAccess:   (-3, -3)
SecondAccess:  (-5, -5)
                       
FirstAccess:   (-7, -7)
SecondAccess:  (-9, -9)

For this reason, if you want to use an object from the top of a PriorityQueue, you should assign that object to a name:

from queue import PriorityQueue          
import threading                         
import time                              
                                         
pq = PriorityQueue()                     
                                         
def queue_manip():                       
    i = 0                                
    while True:                          
        pq.put((i, i))                   
        i -= 1                           
        time.sleep(.1)                   
                                         
t1 = threading.Thread(target=queue_manip)
t1.start()                               
                 
while True:                           
    priority, item = pq.queue[0]      
    print('FirstUse:  ', item)        
    time.sleep(.2)  # other processing
    print('SecondUse: ', item)        
    time.sleep(.2)  # other processing
    print()

which will give you:

FirstUse:   0
SecondUse:  0

FirstUse:   -3
SecondUse:  -3

FirstUse:   -7
SecondUse:  -7

Answered By: Stephen C

Can I get an item from a PriorityQueue without removing it yet?

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