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

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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?

python bash
1 Answers

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)
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