python標准庫介紹——32 Queue 模塊詳解

Queue 模塊


``Queue`` 模塊提供了一個線程安全的隊列 (queue) 實現, 如 [Example 3-2 #eg-3-2] 所示. 
你可以通過它在多個線程里安全訪問同個對象.

====Example 3-2. 使用 Queue 模塊====[eg-3-2]

```
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?00 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

*B*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 finished*b*
```

[Example 3-3 #eg-3-3] 展示了如何限制隊列的大小. 如果隊列滿了, 
那么控制主線程 (producer threads) 被阻塞, 等待項目被彈出 (pop off). 

====Example 3-3. 使用限制大小的 Queue 模塊====[eg-3-3]

```
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?00 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

*B*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 finished*b*
```

你可以通過繼承 //Queue// 類來修改它的行為. [Example 3-4 #eg-3-4] 
為我們展示了一個簡單的具有優先級的隊列. 它接受一個元組作為參數, 
元組的第一個成員表示優先級(數值越小優先級越高). 

====Example 3-4. 使用 Queue 模塊實現優先級隊列====[eg-3-4]

```
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

*B*third
second
first*b*
```

[Example 3-5 #eg-3-5] 展示了一個簡單的堆棧 (stack) 實現 
(末尾添加, 頭部彈出, 而非頭部添加, 頭部彈出).

====Example 3-5. 使用 Queue 模塊實現一個堆棧====[eg-3-5]

```
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

*B*third
second
first*b*
```