转自：caspar
整理：机器学习算法与Python学习
segmentfault.com/a/1190000000414339
Python 在程序并行化方面多少有些声名狼藉。撇开技术上的问题，例如线程的实现和 GIL，我觉得错误的教学指导才是主要问题。常见的经典 Python 多线程、多进程教程多显得偏"重"。而且往往隔靴搔痒，没有深入探讨日常工作中最有用的内容。
传统的例子
简单搜索下"Python 多线程教程"，不难发现几乎所有的教程都给出涉及类和队列的例子：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> os <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> PIL <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> PIL <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Image<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SIZE = (<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>,<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SAVE_DIRECTORY = <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> thumbs </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">get_image_paths</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(folder)</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">return</span> (os.path.join(folder, f) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> f <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> os.listdir(folder) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> jpeg </span> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> f)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">create_thumbnail</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(filename)</span>:</span> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im = Image.open(filename)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.thumbnail(SIZE, Image.ANTIALIAS)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> base, fname = os.path.split(filename) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> save_path = os.path.join(base, SAVE_DIRECTORY, fname)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.save(save_path)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> __name__ == <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> __main__ </span>:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> folder = os.path.abspath(<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> 11_18_2013_R000_IQM_Big_Sur_Mon__e10d1958e7b766c3e840 </span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> os.mkdir(os.path.join(folder, SAVE_DIRECTORY))<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> images = get_image_paths(folder)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool = Pool()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.map(creat_thumbnail, images)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.close()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.join()</p>哈，看起来有些像 Java 不是吗？
我并不是说使用生产者/消费者模型处理多线程/多进程任务是错误的（事实上，这一模型自有其用武之地）。只是，处理日常脚本任务时我们可以使用更有效率的模型。
问题在于…
首先，你需要一个样板类；
其次，你需要一个队列来传递对象；
而且，你还需要在通道两端都构建相应的方法来协助其工作（如果需想要进行双向通信或是保存结果还需要再引入一个队列）。
worker 越多，问题越多
按照这一思路，你现在需要一个 worker 线程的线程池。下面是一篇 IBM 经典教程中的例子——在进行网页检索时通过多线程进行加速。
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;">#Example2.py</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />A more realistic thread pool example <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> time <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> threading <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Queue <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> urllib2 <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">class</span> <span class="" style="max-width: 100%;">Consumer(threading.Thread)</span>:</span> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">__init__</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(self, queue)</span>:</span> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> threading.Thread.__init__(self)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> self._queue = queue <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">run</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(self)</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">while</span> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">True</span>: <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> content = self._queue.get() <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> isinstance(content, str) <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">and</span> content == <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> quit </span>:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">break</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> response = urllib2.urlopen(content)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">print</span> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> Bye byes! </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">Producer</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">()</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> urls = [<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org </span>, <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.yahoo.com </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.scala.org </span>, <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.google.com </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># etc.. </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> ]<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> queue = Queue.Queue()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> worker_threads = build_worker_pool(queue, <span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">4</span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> start_time = time.time()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Add the urls to process</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> url <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> urls: <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> queue.put(url) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Add the poison pillv</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> worker <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> worker_threads:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> queue.put(<span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> quit </span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> worker <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> worker_threads:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> worker.join()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">print</span> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> Done! Time taken: {} </span>.format(time.time() - start_time)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">build_worker_pool</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(queue, size)</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> workers = []<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> _ <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> range(size):<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> worker = Consumer(queue)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> worker.start() <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> workers.append(worker)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">return</span> workers<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> __name__ == <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> __main__ </span>:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> Producer()</p>这段代码能正确的运行，但仔细看看我们需要做些什么：构造不同的方法、追踪一系列的线程，还有为了解决恼人的死锁问题，我们需要进行一系列的 join 操作。这还只是开始……
至此我们回顾了经典的多线程教程，多少有些空洞不是吗？样板化而且易出错，这样事倍功半的风格显然不那么适合日常使用，好在我们还有更好的方法。
何不试试 map
map 这一小巧精致的函数是简捷实现 Python 程序并行化的关键。map 源于 Lisp 这类函数式编程语言。它可以通过一个序列实现两个函数之间的映射。
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"> urls = [<span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.yahoo.com </span>, <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.reddit.com </span>]<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> results = map(urllib2.urlopen, urls)</p>上面的这两行代码将 urls 这一序列中的每个元素作为参数传递到 urlopen 方法中，并将所有结果保存到 results 这一列表中。其结果大致相当于：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;">results = []<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> url <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> urls: <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> results.append(urllib2.urlopen(url))</p>map 函数一手包办了序列操作、参数传递和结果保存等一系列的操作。
为什么这很重要呢？这是因为借助正确的库，map 可以轻松实现并行化操作。
在 Python 中有个两个库包含了 map 函数： multiprocessing 和它鲜为人知的子库 multiprocessing.dummy.
这里多扯两句： multiprocessing.dummy？ mltiprocessing 库的线程版克隆？这是虾米？即便在 multiprocessing 库的官方文档里关于这一子库也只有一句相关描述。而这句描述译成人话基本就是说:"嘛，有这么个东西，你知道就成."相信我，这个库被严重低估了！
dummy 是 multiprocessing 模块的完整克隆，唯一的不同在于 multiprocessing 作用于进程，而 dummy 模块作用于线程（因此也包括了 Python 所有常见的多线程限制）。
所以替换使用这两个库异常容易。你可以针对 IO 密集型任务和 CPU 密集型任务来选择不同的库。
动手尝试
使用下面的两行代码来引用包含并行化 map 函数的库：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing.dummy <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">as</span> ThreadPool</p>实例化 Pool 对象：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;">pool = ThreadPool()</p>这条简单的语句替代了 example2.py 中 buildworkerpool 函数 7 行代码的工作。它生成了一系列的 worker 线程并完成初始化工作、将它们储存在变量中以方便访问。
Pool 对象有一些参数，这里我所需要关注的只是它的第一个参数：processes. 这一参数用于设定线程池中的线程数。其默认值为当前机器 CPU 的核数。
一般来说，执行 CPU 密集型任务时，调用越多的核速度就越快。但是当处理网络密集型任务时，事情有有些难以预计了，通过实验来确定线程池的大小才是明智的。
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;">pool = ThreadPool(<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">4</span>) <span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Sets the pool size to 4</span></p>线程数过多时，切换线程所消耗的时间甚至会超过实际工作时间。对于不同的工作，通过尝试来找到线程池大小的最优值是个不错的主意。
创建好 Pool 对象后，并行化的程序便呼之欲出了。我们来看看改写后的 example2.py
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> urllib2 <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing.dummy <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">as</span> ThreadPool <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />urls = [<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org </span>, <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/about/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.onlamp.com/pub/a/python/2003/04/17/metaclasses.html </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/doc/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/download/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/getit/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/community/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> https://wiki.python.org/moin/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://planet.python.org/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> https://wiki.python.org/moin/LocalUserGroups </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/psf/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://docs.python.org/devguide/ </span>,<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> http://www.python.org/community/awards/ </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># etc.. </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> ]<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Make the Pool of workers</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />pool = ThreadPool(<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">4</span>) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Open the urls in their own threads</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># and return the results</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />results = pool.map(urllib2.urlopen, urls)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;">#close the pool and wait for the work to finish </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />pool.close() <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />pool.join()</p>实际起作用的代码只有 4 行，其中只有一行是关键的。map 函数轻而易举的取代了前文中超过 40 行的例子。为了更有趣一些，我统计了不同方法、不同线程池大小的耗时情况。
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># results = [] </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># for url in urls:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># result = urllib2.urlopen(url)</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># results.append(result)</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># # ------- VERSUS ------- # </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># # ------- 4 Pool ------- # </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># pool = ThreadPool(4) </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># results = pool.map(urllib2.urlopen, urls)</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># # ------- 8 Pool ------- # </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># pool = ThreadPool(8) </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># results = pool.map(urllib2.urlopen, urls)</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># # ------- 13 Pool ------- # </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># pool = ThreadPool(13) </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># results = pool.map(urllib2.urlopen, urls)</span></p>结果：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># Single thread: 14.4 Seconds </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># 4 Pool: 3.1 Seconds</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># 8 Pool: 1.4 Seconds</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(117, 113, 94);box-sizing: border-box !important;overflow-wrap: break-word !important;"># 13 Pool: 1.3 Seconds</span></p>很棒的结果不是吗？这一结果也说明了为什么要通过实验来确定线程池的大小。在我的机器上当线程池大小大于 9 带来的收益就十分有限了。
另一个真实的例子
生成上千张图片的缩略图
这是一个 CPU 密集型的任务，并且十分适合进行并行化。
基础单进程版本
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> os <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> PIL <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> PIL <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Image<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SIZE = (<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>,<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SAVE_DIRECTORY = <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> thumbs </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">get_image_paths</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(folder)</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">return</span> (os.path.join(folder, f) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> f <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> os.listdir(folder) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> jpeg </span> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> f)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">create_thumbnail</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(filename)</span>:</span> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im = Image.open(filename)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.thumbnail(SIZE, Image.ANTIALIAS)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> base, fname = os.path.split(filename) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> save_path = os.path.join(base, SAVE_DIRECTORY, fname)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.save(save_path)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> __name__ == <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> __main__ </span>:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> folder = os.path.abspath(<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> 11_18_2013_R000_IQM_Big_Sur_Mon__e10d1958e7b766c3e840 </span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> os.mkdir(os.path.join(folder, SAVE_DIRECTORY))<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> images = get_image_paths(folder)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> image <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> images:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> create_thumbnail(Image)</p>上边这段代码的主要工作就是将遍历传入的文件夹中的图片文件，一一生成缩略图，并将这些缩略图保存到特定文件夹中。
这我的机器上，用这一程序处理 6000 张图片需要花费 27.9 秒。
如果我们使用 map 函数来代替 for 循环：
<p style="padding: 0.5em;max-width: 100%;font-size: 0.85em;font-family: Consolas, Inconsolata, Courier, monospace;overflow: auto;border-radius: 3px;border-width: 1px;border-style: solid;border-color: rgb(204, 204, 204);color: rgb(248, 248, 242);background: rgb(35, 36, 31);margin-left: 16px;margin-right: 16px;box-sizing: border-box !important;overflow-wrap: break-word !important;display: block !important;"><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> os <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> PIL <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> multiprocessing <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Pool <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">from</span> PIL <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">import</span> Image<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SIZE = (<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>,<span class="" style="max-width: 100%;color: rgb(174, 129, 255);box-sizing: border-box !important;overflow-wrap: break-word !important;">75</span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" />SAVE_DIRECTORY = <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> thumbs </span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">get_image_paths</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(folder)</span>:</span><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">return</span> (os.path.join(folder, f) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">for</span> f <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> os.listdir(folder) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> jpeg </span> <span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">in</span> f)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;"><span class="" style="max-width: 100%;color: rgb(102, 217, 239);">def</span> <span class="" style="max-width: 100%;color: rgb(166, 226, 46);">create_thumbnail</span><span class="" style="max-width: 100%;color: rgb(248, 248, 242);">(filename)</span>:</span> <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im = Image.open(filename)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.thumbnail(SIZE, Image.ANTIALIAS)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> base, fname = os.path.split(filename) <br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> save_path = os.path.join(base, SAVE_DIRECTORY, fname)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> im.save(save_path)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><span class="" style="max-width: 100%;color: rgb(249, 38, 114);box-sizing: border-box !important;overflow-wrap: break-word !important;">if</span> __name__ == <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> __main__ </span>:<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> folder = os.path.abspath(<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> <span class="" style="max-width: 100%;color: rgb(230, 219, 116);box-sizing: border-box !important;overflow-wrap: break-word !important;"> 11_18_2013_R000_IQM_Big_Sur_Mon__e10d1958e7b766c3e840 </span>)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> os.mkdir(os.path.join(folder, SAVE_DIRECTORY))<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> images = get_image_paths(folder)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /><br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool = Pool()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.map(creat_thumbnail, images)<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.close()<br style="max-width: 100%;box-sizing: border-box !important;overflow-wrap: break-word !important;" /> pool.join()</p>5.6 秒！
虽然只改动了几行代码，我们却明显提高了程序的执行速度。在生产环境中，我们可以为 CPU 密集型任务和 IO 密集型任务分别选择多进程和多线程库来进一步提高执行速度——这也是解决死锁问题的良方。此外，由于 map 函数并不支持手动线程管理，反而使得相关的 debug 工作也变得异常简单。
到这里，我们就实现了（基本）通过一行 Python 实现并行化。
《认认真真推荐》
推荐阅读
过节啦收礼物，送给大朋友们精选Python书20本
漫画，杨幂和杨超越到底谁更美
漫画，假装自己是Python高手，学会这十招就可以
本篇文章来源于: 菜鸟学Python
本文为原创文章，版权归知行编程网所有，欢迎分享本文，转载请保留出处！
内容反馈