Queue Module

Introduction

The queue module provides thread-safe queue implementations: Queue (FIFO), LifoQueue (LIFO), and PriorityQueue for organized message passing.

Queue (FIFO)

import queue
import threading

q = queue.Queue()

def producer():
    for i in range(10):
        q.put(i)
    q.put(None)  # Sentinel

def consumer():
    while True:
        item = q.get()
        if item is None:
            break
        print(f"Got: {item}")
        q.task_done()

threading.Thread(target=producer).start()
threading.Thread(target=consumer).start()

LifoQueue (Stack)

import queue

stack = queue.LifoQueue()

stack.put(1)
stack.put(2)
stack.put(3)

print(stack.get())  # 3 (last in)
print(stack.get())  # 2
print(stack.get())  # 1

PriorityQueue

import queue

class Task:
    def __init__(self, priority, description):
        self.priority = priority
        self.description = description
    
    def __lt__(self, other):
        return self.priority < other.priority

pq = queue.PriorityQueue()

pq.put(Task(3, "Low priority"))
pq.put(Task(1, "High priority"))
pq.put(Task(2, "Medium priority"))

while not pq.empty():
    task = pq.get()
    print(task.description)

Thread-Safe Operations

import queue

q = queue.Queue(maxsize=10)

# Safe operations
q.put(item, block=True, timeout=5)
q.put_nowait(item)  # Non-blocking

item = q.get(block=True, timeout=5)
item = q.get_nowait()  # Non-blocking

q.empty()  # Check if empty
q.full()   # Check if full
q.qsize()  # Approximate size

Queue with Multiple Producers

import queue
import threading
import time

q = queue.Queue()

def worker(n):
    while True:
        item = q.get()
        if item is None:
            q.put(None)  # Pass sentinel to next worker
            break
        print(f"Worker {n} processed {item}")
        q.task_done()

# Start multiple workers
threads = [threading.Thread(target=worker, args=(i,)) for i in range(3)]
for t in threads:
    t.start()

# Produce items
for i in range(10):
    q.put(i)
q.put(None)  # Signal end

for t in threads:
    t.join()

Practice Problems

1. Implement producer-consumer with Queue
2. Use LifoQueue for undo functionality
3. Implement PriorityQueue for task scheduling
4. Handle Queue exceptions properly
5. Build multi-threaded data pipeline