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POWhy is subprocess.Popen not waiting until the child process terminates?
 

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  1. POWhy is subprocess.Popen not waiting until the child process terminates?
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    <p>I'm having a problem with Python's subprocess.Popen method. </p> <p>Here's a test script which demonstrates the problem. It's being run on a Linux box.</p> <pre><code>#!/usr/bin/env python import subprocess import time def run(cmd): p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE) return p ### START MAIN # copy some rows from a source table to a destination table # note that the destination table is empty when this script is run cmd = 'mysql -u ve --skip-column-names --batch --execute="insert into destination (select * from source limit 100000)" test' run(cmd) # check to see how many rows exist in the destination table cmd = 'mysql -u ve --skip-column-names --batch --execute="select count(*) from destination" test' process = run(cmd) count = (int(process.communicate()[0][:-1])) # if subprocess.Popen() waited for the child to terminate than count should be # greater than 0 if count &gt; 0: print "success: " + str(count) else: print "failure: " + str(count) time.sleep(5) # find out how many rows exists in the destination table after sleeping process = run(cmd) count = (int(process.communicate()[0][:-1])) print "after sleeping the count is " + str(count) </code></pre> <p>Usually the output from this script is:</p> <pre><code>success: 100000 </code></pre> <p>but sometimes it's</p> <pre><code>failure: 0 after sleeping the count is 100000 </code></pre> <p>Note that in the failure case, the select immediately after the insert shows 0 rows but after sleeping for 5 seconds a second select correctly shows a row count of 100000. My conclusion is that one of the following is true:</p> <ol> <li>subprocess.Popen is not waiting for the child thread to terminate - This seems to contradict the documentation</li> <li>the mysql insert is not atomic - my understanding of mysql seems to indicate insert is atomic</li> <li>the select is not seeing the correct row count right away - according to a friend who knows mysql better than I do this should not happen either</li> </ol> <p>What am I missing?</p> <p>FYI, I'm aware that this is a hacky way of interacting with mysql from Python and MySQLdb would likely not have this problem but I'm curious as to why this method does not work.</p>
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    1. COThanks everyone for the great answers. Looking at the subprocess documentation again I see that I was thrown off by the comment "Wait for command to complete" which appears in the convenience methods sections not the Popen method section. I gave the nod to Jed's answer since it best answered my original question though I think I'll use Paul's solution for my future scripting needs.
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    2. COKeep in mind that os.system (unless you do something else with it) returns the RETURN VALUE of the process (usually 0 or 1). Don't let that bite you either.
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      1. USPaul McMillan
 

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