How can I get full output error if subprocess.run failed?

Question

For example I'm trying to run some bash command from python:

from subprocess import run

command = f'ffmpeg -y -i "{video_path}" "{frames_path}/%d.png"'
run(command, shell=True, check=True)

but if it fails I just get subprocess.CalledProcessError: Command 'ffmpeg ...' returned non-zero exit status 127. how can I get full ffmpeg error message?

I would probably start with a review of the documentation. You need (a) to instruct subprocess.run to capture the output in which you are interested, and then (b) inspect the value returned by run to retrieve the data you want. — larsks
– larsks, Commented Jan 13, 2022 at 17:17

Gbox4 · Accepted Answer · 2022-01-13 21:28:53Z

1

It's the check=True kwarg that's causing it to throw a CalledProcessError. Just remove check=True, and it will stop throwing the error. If you want to print the STDERR printed by ffmpeg, you can use capture_output=True. Then, the resulting CompletedProcess object will have a .stderr member that contains the STDERR of your command, encoded as a bytes-like string. Use str.decode() to turn it into a normal string:

from subprocess import run

command = f'ffmpeg -y -i "{video_path}" "{frames_path}/%d.png"'
proc = run(command, shell=True, capture_output=True)
out = proc.stdout.decode() # stores the output of stdout
err = proc.stderr.decode() # stores the output of stderr
print(err)

answered Jan 13, 2022 at 21:28

Gbox4

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