The Queue module provides a thread-safe queue implementation, shown in Example 3-2. It provides
a convenient way of moving Python objects between different threads.
Example 3-2. Using the Queue Module
File: queue-example-1.py
import threading
import Queue
import time, random
WORKERS = 2
class Worker(threading.Thread):
def _ _init_ _(self, queue):
self._ _queue = queue
threading.Thread._ _init_ _(self)
def run(self):
while 1:
item = self._ _queue.get()
if item is None:
break # reached end of queue
# pretend we're doing something that takes 10—100 ms
time.sleep(random.randint(10, 100) / 1000.0)
print "task", item, "finished"
#
# try it
queue = Queue.Queue(0)
for i in range(WORKERS):
Worker(queue).start() # start a worker
for i in range(10):
queue.put(i)
for i in range(WORKERS):
queue.put(None) # add end-of-queue markers
task 1 finished
task 0 finished
task 3 finished
task 2 finished
task 4 finished
task 5 finished
task 7 finished
task 6 finished
task 9 finished
task 8 finishedExample 3-3 shows how you can limit the size of the queue. If the producer threads fill the queue, they will block until items are popped off the queue.
Example 3-3. Using the Queue Module with a Maximum Size
File: queue-example-2.py
import threading
import Queue
import time, random
WORKERS = 2
class Worker(threading.Thread):
def _ _init_ _(self, queue):
self._ _queue = queue
threading.Thread._ _init_ _(self)
def run(self):
while 1:
item = self._ _queue.get()
if item is None:
break # reached end of queue
# pretend we're doing something that takes 10—100 ms
time.sleep(random.randint(10, 100) / 1000.0)
print "task", item, "finished"
#
# run with limited queue
queue = Queue.Queue(3)
for i in range(WORKERS):
Worker(queue).start() # start a worker
for item in range(10):
print "push", item
queue.put(item)
for i in range(WORKERS):
queue.put(None) # add end-of-queue markers
push 0
push 1
push 2
push 3
push 4
push 5
task 0 finished
push 6
task 1 finished
push 7
task 2 finished
push 8
task 3 finished
push 9
task 4 finished
task 6 finished
task 5 finished
task 7 finished
task 9 finished
task 8 finishedYou can modify the behavior through subclassing. The class in Example 3-4 provides a simple priority queue. It expects all items added to the queue to be tuples, where the first member contains the priority (lower value means higher priority).
Example 3-4. Using the Queue Module to Implement a Priority Queue
File: queue-example-3.py
import Queue
import bisect
Empty = Queue.Empty
class PriorityQueue(Queue.Queue):
"Thread-safe priority queue"
def _put(self, item):
# insert in order
bisect.insort(self.queue, item)
#
# try it
queue = PriorityQueue(0)
# add items out of order
queue.put((20, "second"))
queue.put((10, "first"))
queue.put((30, "third"))
# print queue contents
try:
while 1:
print queue.get_nowait()
except Empty:
pass
third
second
firstExample 3-5 shows a simple stack implementation (last-in, first-out, instead of first-in, first-out).
Example 3-5. Using the Queue Module to Implement a Stack
File: queue-example-4.py
import Queue
Empty = Queue.Empty
class Stack(Queue.Queue):
"Thread-safe stack"
def _put(self, item):
# insert at the beginning of queue, not at the end
self.queue.insert(0, item)
# method aliases
push = Queue.Queue.put
pop = Queue.Queue.get
pop_nowait = Queue.Queue.get_nowait
#
# try it
stack = Stack(0)
# push items on stack
stack.push("first")
stack.push("second")
stack.push("third")
# print stack contents
try:
while 1:
print stack.pop_nowait()
except Empty:
pass
third
second
first