Python Queue Module
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.
Importing the Module
import queueFIFO Queue (Queue)
The Queue class implements a basic first-in, first-out queue.
import queue
q = queue.Queue()
q.put(1)
q.put(2)
q.put(3)
print(q.get()) # Output: 1
print(q.get()) # Output: 2
print(q.get()) # Output: 3LIFO Queue (LifoQueue)
The LifoQueue class implements a last-in, first-out queue (like a stack).
import queue
q = queue.LifoQueue()
q.put(1)
q.put(2)
q.put(3)
print(q.get()) # Output: 3
print(q.get()) # Output: 2
print(q.get()) # Output: 1Priority Queue (PriorityQueue)
The PriorityQueue class retrieves entries in priority order (lowest first). Entries are typically tuples of the form (priority_number, data).
import queue
q = queue.PriorityQueue()
q.put((2, 'code'))
q.put((1, 'eat'))
q.put((3, 'sleep'))
while not q.empty():
next_item = q.get()
print(next_item)
# Output:
# (1, 'eat')
# (2, 'code')
# (3, 'sleep')Blocking and Timeouts
put() and get() block by default if the queue is full (for put) or empty (for get). You can specify a timeout.
try:
item = q.get(timeout=1)
except queue.Empty:
print("Queue is empty")Task Tracking
task_done() and join() are used to track when enqueued tasks are completed.
import threading
import queue
def worker():
while True:
item = q.get()
print(f'Working on {item}')
print(f'Finished {item}')
q.task_done()
q = queue.Queue()
# Turn-on the worker thread
threading.Thread(target=worker, daemon=True).start()
# Send thirty task requests to the worker
for item in range(30):
q.put(item)
# Block until all tasks are done
q.join()
print('All work completed')