Python data structure for auto-requeueing queue

Question

Are there any standard library data structures for a queue that automatically requeues to the opposite end of the queue when an item is popped off the end? This feels like a common problem so I imagine that there could be a simple data structure that does this.

For example:

from collections import deque
from time import sleep

queue = deque([1, 2, 3, 4, 5, 6, 7, 8])

while True:
    item = queue.pop()
    queue.appendleft(item)
    print(item)
    sleep(5)

Is the above code actually optimal, or is there a better way to solve this problem?
Would it be better to just use a list and modify an index value each iteration of the loop to change which place in the list is accessed?

RootTwo · Accepted Answer · 2019-08-18 21:09:57Z

1

deque.rotate()

`collections.deque' can be indexed like a list and has a rotate method.

from collections import deque
from time import sleep

queue = deque([1, 2, 3, 4, 5, 6, 7, 8])

while True:
    item = queue[0]
    queue.rotate(1)
    print(item)
    sleep(5)

answered Aug 18, 2019 at 21:09

RootTwo

4,4361 gold badge13 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

What you're doing is called a rotation. Deque has this as a builtin: deque.rotate(n).

Rotate the deque n steps to the right. If n is negative, rotate to the left.

When the deque is not empty, rotating one step to the right is equivalent to d.appendleft(d.pop()), and rotating one step to the left is equivalent to d.append(d.popleft()).

Usage:

>>> from collections import deque
>>> dq = deque()
>>> dq.append(4)
>>> dq.append(5)
>>> dq.append(6)
>>> dq
deque([4, 5, 6])
>>> dq.rotate()
>>> dq
deque([6, 4, 5])
>>> dq.rotate(2)
>>> dq
deque([4, 5, 6])
>>> dq.rotate(-2)
>>> dq
deque([6, 4, 5])

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Aug 18, 2019 at 21:09

ggorlen

59.3k8 gold badges119 silver badges173 bronze badges

Comments

Tammo Heeren · Accepted Answer · 2019-08-18 21:09:39Z

1

Check out

https://docs.python.org/2/library/itertools.html#itertools.cycle

def cycle(iterable):
    # cycle('ABCD') --> A B C D A B C D A B C D ...
    saved = []
    for element in iterable:
        yield element
        saved.append(element)
    while saved:
        for element in saved:
              yield element

This should probably do what you need, and be very efficient.

answered Aug 18, 2019 at 21:09

Tammo Heeren

2,1043 gold badges16 silver badges21 bronze badges

Collectives™ on Stack Overflow

Python data structure for auto-requeueing queue

3 Answers 3

deque.rotate()

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

deque.rotate()

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related