How to hide output of subprocess

python

subprocess

error-handling

process-management

byNikita Barsukov·Feb 18, 2025

To silence a Python subprocess, set both stdout and stderr to subprocess.DEVNULL:

import subprocess

subprocess.run(['command'], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

This swallows all output, discarding it into the void.

For our friends still on Python < 3.3, you'll need to redirect output to an os.devnull file object:

import os
import subprocess

FNULL = open(os.devnull, 'w')
subprocess.run(['command'], stdout=FNULL, stderr=FNULL)
FNULL.close()  # Always remember to clean up after yourself!

Merging stdout and stderr

If you need to combine the stdout and stderr and also silence them:

subprocess.run(['command'], stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT)

Doing this is akin to dumping both your electronic waste and household waste into the same bin. Efficient, right?

Gracefully handling exceptions

Remember, not all commands execute successfully. Wrap your calls with try-except blocks:

try:
    subprocess.run(['command'], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
except subprocess.CalledProcessError as e:
    print(f"Oops! Error in execution: {e}")  # Murphy's law in action

This ensures your script acts like a cat, landing on its feet, even when the subprocess fails.

Mastering subprocess with Popen

For those who crave granular control, subprocess.Popen is a virtuoso:

with subprocess.Popen(['command'], stdout=subprocess.DEVNULL, stderr=subprocess.PIPE) as proc:
    errors = proc.stderr.read()  # In case you miss the angry error messages

This ensures you can separate and process error information, like mulling over why your code doesn't cooperate.

Sneakily capturing output

To capture output for later use without making it public right away, put it in a paper plane:

result = subprocess.run(['command'], stdout=subprocess.PIPE, stderr=subprocess.PIPE) # Clever, isn't it?
output = result.stdout.decode('utf-8') # Decoding secrets

Tiptoeing around deprecation

For the love of Python, avoid using deprecated commands.getoutput(), it's been sent to retirement. Stick to the subprocess module with its bells and whistles.

Compatibility and portability

Ensuring scripts run smoothly on both Python 2 and 3:

Stick to subprocess.Popen for a safe passage.
Decode outputs or use universal_newlines=True for managing those encoding hassles.

Robust error handling

Make your scripts bulletproof with comprehensive error handling:

Use exit codes to ensure operations were successful. It's like checking all passengers are on board before the flight.
Log errors to file for those intense debugging sessions.

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