Subprocess testing in PyTest

Introduction

Subprocess testing helps check if programs run other programs correctly. It makes sure the commands and outputs are right.

When your program runs a shell command and you want to check its output.

When you want to test if an external script or tool is called properly.

When you need to verify error messages from a subprocess.

When you want to check the return code of a command your program runs.

When you want to simulate running commands without actually running them.

Syntax

PyTest

import subprocess

result = subprocess.run(['command', 'arg1', 'arg2'], capture_output=True, text=True)
print(result.stdout)
print(result.returncode)

subprocess.run() runs a command and waits for it to finish.

capture_output=True captures the output so you can check it.

Examples

This runs the echo hello command and prints its output.

PyTest

import subprocess

result = subprocess.run(['echo', 'hello'], capture_output=True, text=True)
print(result.stdout)

This tries to list a missing folder, then prints the error message and return code.

PyTest

import subprocess

result = subprocess.run(['ls', '/nonexistent'], capture_output=True, text=True)
print(result.stderr)
print(result.returncode)

This checks the Python version by running python --version.

PyTest

import subprocess

result = subprocess.run(['python', '--version'], capture_output=True, text=True)
print(result.stdout.strip())

Sample Program

This test runs the echo hello command and checks if it returns 0 (success) and outputs 'hello'.

PyTest

import subprocess

def test_echo_hello():
    result = subprocess.run(['echo', 'hello'], capture_output=True, text=True)
    assert result.returncode == 0
    assert result.stdout.strip() == 'hello'

if __name__ == '__main__':
    test_echo_hello()
    print('Test passed')

Important Notes

Always use capture_output=True and text=True to get readable output.

Check returncode to know if the command succeeded (0 means success).

Use strip() to remove extra spaces or newlines from output before asserting.

Summary

Subprocess testing checks commands run by your program.

Use subprocess.run() with capture_output=True to get output.

Assert on stdout, stderr, and returncode to verify behavior.