26. Running Commands with subprocess

The subprocess module lets Python run external commands. This is useful when you want to automate shell tools, capture output, or check whether a command succeeded.

26.1. Running a command

The simplest pattern is subprocess.run() with a list of arguments.

import subprocess

subprocess.run(['python3', '-c', "print('hello from subprocess')"])

26.2. Capturing output

If you need the command output inside Python, capture it explicitly.

import subprocess

result = subprocess.run(
    ['python3', '-c', "print('captured output')"],
    capture_output=True,
    text=True,
    check=True,
)

print(result.stdout.strip())

26.3. Checking failures

Use check=True if a non-zero exit status should raise an exception.

import subprocess

try:
    subprocess.run(
        ['python3', '-c', 'import sys; sys.exit(1)'],
        check=True,
    )
except subprocess.CalledProcessError as exc:
    print(f'command failed with status {exc.returncode}')

26.4. Exercise

Write a script that runs two external commands: one that succeeds and one that fails. Capture the output of the successful command and catch the exception from the failing one.

26.4.1. Solution

import subprocess

ok = subprocess.run(['python3', '-c', "print('hello')"], capture_output=True, text=True, check=True)
print(ok.stdout.strip())

try:
    subprocess.run(['python3', '-c', 'import sys; sys.exit(1)'], check=True)
except subprocess.CalledProcessError as exc:
    print(f'command failed: {exc.returncode}')