python subprocess communicate() block

python subprocess blocking
Mingo picture Mingo · Jan 25, 2010 · Viewed 23.5k times · Source

I am using the subprocess module to call an external program (plink.exe) to log-in to a server; but when I call communicate to read the output, it is blocking. The code is below:

 import subprocess
 process = subprocess.Popen('plink.exe [email protected] -pw 123456'.split(), shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
 print process.communicate() #block here

I know the block is because plink.exe it still running; but I need to read the output before the subprocess terminates. Is there anyway to do that?

Answer

Thomas Wouters picture Thomas Wouters · Jan 25, 2010

The whole purpose of the communicate method is to wait for the process to finish and return all the output. If you don't want to wait, don't call communicate. Instead, read from the stdout or stderr attribute to read the output.

If the process outputs to both stdout and stderr (and you want to read it separately), you will have to be careful to actually read from both without blocking, or you can deadlock. This is fairly hard on Windows, and you may wish to use the pexpect module instead.

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