如何处理优先级队列中具有相同优先级的项目？ （蟒蛇）

Question

I am trying to create a priority queue, which processes input in this form:

enqueue 2 2
enqueue 1 3
enqueue 5 1
enqueue 6 2
dequeue
dequeue
dequeue
dequeue

I have this code that uses lists:

from queue import Queue
data=[]
while True: 
    try:
        operation = input()
    except:
        break
    data.append(operation.split(" "))

data2=[] 
data3=[]
for i in data: #split enqueue and dequeue as not to get an out of range error
    if "enqueue" in i:
        data2.append(i)
    else:
        data3.append(i)

data2=sorted(data2,key=lambda x: int(x[2]))
data2=data2[::-1]

data=data2+data3 #merged the two again after sorting

q=[]

for i in data:
    if "enqueue" in i: #add the item by the already sorted order of priority
        q.append(i[1])
    if "dequeue" in i: #print the first item dequeued before removing it from the queue
        print (q[0])
        del q[0]

My problem with this is that it outputs:

1
6
2
5

Instead of: (since 2 was technically enqueued before 6, and in the case of items having the same priority, they need to follow the FIFO structure of queues)

1
2
6
5

Does anyone have any idea on how I can fix this? Is there an alternate way to solve this using queues instead of lists? Thanks!

Answer 1

If you print out 'data2' before sort, after sort, and after data2=data2[::-1] you will see why this is occurring.

[['enqueue', '2', '2'], ['enqueue', '1', '3'], ['enqueue', '5', '1'], ['enqueue', '6', '2']]
[['enqueue', '5', '1'], ['enqueue', '2', '2'], ['enqueue', '6', '2'], ['enqueue', '1', '3']]
[['enqueue', '1', '3'], ['enqueue', '6', '2'], ['enqueue', '2', '2'], ['enqueue', '5', '1']]

Before you flip it, the sort is just right (ie: elements with the same key are in the order they were input), but reverse of what you want gives you those lists flipped.

Instead of:

data2=sorted(data2,key=lambda x: int(x[2]))
data2=data2[::-1]

Try:

data2=sorted(data2,key=lambda x: int(x[2]), reverse=True)