【Python有坑系列】python多进程，函数内print的内容没有打印出

发布时间：2020-12-17 01:23:08 所属栏目：Python 来源：网络整理

导读：问题：python多进程，子函数内容没有打印出来。 Simple Python Multiprocessing function doesn't output results I have this very simple function right here in which I'm trying to run and test on,however,it doesn't output anything and it doesn't

问题：python多进程，子函数内容没有打印出来。

Simple Python Multiprocessing function doesn't output results

I have this very simple function right here in which I'm trying to run and test on,however,it doesn't output anything and it doesn't have any errors either. I've checked the code multiple times but it doesn't have any errors.

I printed jobs and here's what I got:

<pre class="lang-py prettyprint prettyprinted">
[<Process(Process-12, stopped[1])>, <Process(Process-13, <Process(Process-14, <Process(Process-15, <Process(Process-16, stopped[1])>]

Here's the code:

<pre class="lang-py prettyprint prettyprinted">
import multiprocessing


<span style="color:#101094;">def<span style="color:#303336;"> worker<span style="color:#303336;">(<span style="color:#303336;">num<span style="color:#303336;">):<span style="color:#303336;">

<span style="color:#101094;">print<span style="color:#303336;"> <span style="color:#7d2727;">"worker "<span style="color:#303336;">,<span style="color:#303336;"> num

<span style="color:#101094;">return<span style="color:#303336;">

jobs = [] for i in range(5): p = multiprocessing.Process(target = worker, args = (i,)) jobs.append(p) p.start()

Here's the result I'm expecting but it's not outputting anything:

<pre class="lang-py prettyprint prettyprinted">
Worker: 0 Worker: 1 Worker: 2 Worker: 3 Worker: 4

原因：spyder使用的stdout和windows不支持forking，所以无法打印子进程内容。

stdoutstdoutsys.__stdout__

There are two alternatives:

Using the??module. This would encompass creating and logging all messages to one or several files. Using a single log-file may lead to the problem that the output is slightly garbled since the processes would write concurrently to the file. Using a single file per process could solve this.

Not using?print?within the child processes,but simply returning the result to the main process. Either by using a??(or?multiprocessing.Manager().Queue()?since forking is not possible) or more simply by relying on the??map?functionality,see example below.

Multiprocessing example with a Pool:

<pre class="lang-py prettyprint prettyprinted">
import multiprocessing


<span style="color:#101094;">def<span style="color:#303336;"> worker<span style="color:#303336;">(<span style="color:#303336;">num<span style="color:#303336;">):<span style="color:#303336;">

<span style="color:#7d2727;">"""Returns the string of interest"""<span style="color:#303336;">

<span style="color:#101094;">return<span style="color:#303336;"> <span style="color:#7d2727;">"worker %d"<span style="color:#303336;"> <span style="color:#303336;">%<span style="color:#303336;"> num
<span style="color:#101094;">def<span style="color:#303336;"> main<span style="color:#303336;">():<span style="color:#303336;">

pool <span style="color:#303336;">=<span style="color:#303336;"> multiprocessing<span style="color:#303336;">.<span style="color:#2b91af;">Pool<span style="color:#303336;">(<span style="color:#7d2727;">4<span style="color:#303336;">)<span style="color:#303336;">

results <span style="color:#303336;">=<span style="color:#303336;"> pool<span style="color:#303336;">.<span style="color:#303336;">map<span style="color:#303336;">(<span style="color:#303336;">worker<span style="color:#303336;">,<span style="color:#303336;"> range<span style="color:#303336;">(<span style="color:#7d2727;">10<span style="color:#303336;">))<span style="color:#303336;">
pool</span><span style="color:#303336;"&gt;.</span><span style="color:#303336;"&gt;close</span><span style="color:#303336;"&gt;()</span><span style="color:#303336;"&gt;
pool</span><span style="color:#303336;"&gt;.</span><span style="color:#303336;"&gt;join</span><span style="color:#303336;"&gt;()</span><span style="color:#303336;"&gt;

</span><span style="color:#101094;"&gt;for</span><span style="color:#303336;"&gt; result </span><span style="color:#101094;"&gt;in</span><span style="color:#303336;"&gt; results</span><span style="color:#303336;"&gt;:</span><span style="color:#303336;"&gt;
    </span><span style="color:#858c93;"&gt;# prints the result string in the main process</span><span style="color:#303336;"&gt;
    </span><span style="color:#101094;"&gt;print</span><span style="color:#303336;"&gt;(</span><span style="color:#303336;"&gt;result</span><span style="color:#303336;"&gt;)</span><span style="color:#303336;"&gt;

if name == 'main': # Better protect your main function when you use multiprocessing main()

which prints (in the main process)

<pre class="lang-py prettyprint prettyprinted">
worker 0 worker 1 worker 2 worker 3 worker 4 worker 5 worker 6 worker 7 worker 8 worker 9

EDIT: If you are to impatient to wait for the?map?function to finish,you can immediately print your results by using?imap_unordered?and slightly changing the order of the commands:

<pre class="lang-py prettyprint prettyprinted">
def main(): pool = multiprocessing.Pool(4) results = pool.imap_unordered(worker, range(10))


</span><span style="color:#101094;"&gt;for</span><span style="color:#303336;"&gt; result </span><span style="color:#101094;"&gt;in</span><span style="color:#303336;"&gt; results</span><span style="color:#303336;"&gt;:</span><span style="color:#303336;"&gt;
    </span><span style="color:#858c93;"&gt;# prints the result string in the main process as soon as say are ready</span><span style="color:#303336;"&gt;
    </span><span style="color:#858c93;"&gt;# but results are now no longer in order!</span><span style="color:#303336;"&gt;
    </span><span style="color:#101094;"&gt;print</span><span style="color:#303336;"&gt;(</span><span style="color:#303336;"&gt;result</span><span style="color:#303336;"&gt;)</span><span style="color:#303336;"&gt;

</span><span style="color:#858c93;"&gt;# The pool should join after printing all results</span><span style="color:#303336;"&gt;
pool</span><span style="color:#303336;"&gt;.</span><span style="color:#303336;"&gt;close</span><span style="color:#303336;"&gt;()</span><span style="color:#303336;"&gt;
pool</span><span style="color:#303336;"&gt;.</span><span style="color:#303336;"&gt;join</span><span style="color:#303336;"&gt;()</span></code></pre>
来源：https://stackoverflow.com/questions/29629103/simple-python-multiprocessing-function-doesnt-output-results/29632397#29632397
                        （编辑：李大同）
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