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

发布时间：2020-05-27 04:32:27 所属栏目：Python 来源：互联网

导读：问题：python多进程，子函数内容没有打印出来。SimplePythonMultiprocessingfunctiondoesn'toutputresults

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 themodule. 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 usingprintwithin the child processes,but simply returning the result to the main process. Either by using a(ormultiprocessing.Manager().Queue()since forking is not possible) or more simply by relying on themapfunctionality,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;
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