Dumping queue into list or array in Python

A queue is a linear data structure that follows the FIFO (First In, First Out) principle. While queues only allow access to the front element, sometimes we need to convert the entire queue into a list for easier manipulation. Python provides multiple approaches to dump queue contents into a list.

Creating a Queue in Python

Python offers two main queue implementations: collections.deque and queue.Queue. Let's first see how a basic queue works ?

from collections import deque 

# Create and populate a queue
queue = deque()
queue.append(1)
queue.append(2)
queue.append(3)
queue.append(4)

print("Queue contents:", queue)

# Remove first element (FIFO)
first_element = queue.popleft()
print("Removed element:", first_element)
print("Queue after removal:", queue)

Queue contents: deque([1, 2, 3, 4])
Removed element: 1
Queue after removal: deque([2, 3, 4])

Method 1: Using collections.deque

The deque (doubleended queue) can be easily converted to a list using the list() constructor ?

from collections import deque 

# Create and populate the queue
queue = deque([10, 20, 30, 40, 50])
print("Original queue:", queue)
print("Queue type:", type(queue))

# Convert queue to list
queue_list = list(queue)
print("Converted to list:", queue_list)
print("List type:", type(queue_list))

# Original queue remains unchanged
print("Original queue after conversion:", queue)

Original queue: deque([10, 20, 30, 40, 50])
Queue type: <class 'collections.deque'>
Converted to list: [10, 20, 30, 40, 50]
List type: <class 'list'>
Original queue after conversion: deque([10, 20, 30, 40, 50])

Method 2: Using queue.Queue

The queue.Queue class is threadsafe but requires accessing the internal .queue attribute for conversion ?

from queue import Queue

# Create and populate the queue
q = Queue()
q.put('A')
q.put('B')
q.put('C')
q.put('D')

print("Queue contents:", q.queue)
print("Queue type:", type(q))

# Convert to list using the internal queue attribute
queue_list = list(q.queue)
print("Converted to list:", queue_list)
print("List type:", type(queue_list))

Queue contents: deque(['A', 'B', 'C', 'D'])
Queue type: <class 'queue.Queue'>
Converted to list: ['A', 'B', 'C', 'D']
List type: <class 'list'>

Method 3: Draining Queue Contents

For threadsafe operations, you can drain the queue contents while building a list ?

from queue import Queue

# Create and populate the queue
q = Queue()
for item in [100, 200, 300, 400]:
    q.put(item)

print("Queue size before draining:", q.qsize())

# Drain queue into list
queue_list = []
while not q.empty():
    queue_list.append(q.get())

print("Drained list:", queue_list)
print("Queue size after draining:", q.qsize())

Queue size before draining: 4
Drained list: [100, 200, 300, 400]
Queue size after draining: 0

Comparison

Conclusion

Use list(deque) for simple conversions in singlethreaded applications. For threadsafe operations, drain the queue using get() in a loop. Both approaches have O(n) time and space complexity.