Python subprocess returning output as stderr

Question

I've been working on a Python script to interact with ffmpeg; however, I've noticed that even though everything runs fine, stdout is empty and stderr returns what I should expect from stdout. How do I fix it so that the output will be returned by stdout?

Here's the simplest example that reproduces the phenomenon:

from subprocess import Popen, PIPE

p = Popen(['python', '-V'], stdin=PIPE, stdout=PIPE, stderr=PIPE)
out, err = p.communicate()
if out:
    print("Output. ", out.decode())
else:
    print("Error. ", err.decode())

Here's the output:

Error.  Python 3.6.1 :: Anaconda 4.4.0 (64-bit)

I should note that I'm using Windows 10.

What is your question here ?

Xatyrian
– Xatyrian

2017-08-01 09:14:49 +00:00
Commented Aug 1, 2017 at 9:14 — Xatyrian
– Xatyrian, Commented Aug 1, 2017 at 9:14

TDk · Accepted Answer · 2017-08-01 09:53:19Z

2

You can redirect the stderr of your process to its stdoutby doing so:

from subprocess import PIPE, STDOUT
p = subprocess.Popen(["python", "-V"], stdout=PIPE, stderr=STDOUT)

Then you can retrieve the output produced by the process like so:

out = p.stdout.read()

This will return the content of the stdout after your process has terminated.

answered Aug 1, 2017 at 9:53

TDk

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