Subprocess testing in PyTest - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of subprocess.run with capture

What is the output of this pytest test when running the subprocess call?

PyTest

import subprocess

def test_echo():
    result = subprocess.run(['echo', 'hello'], capture_output=True, text=True)
    assert result.stdout == 'hello\n'
    print(result.stdout)

A hello

B hello\n

C ['hello']

D None

💡 Hint

Remember that echo adds a newline and capture_output=True captures stdout as bytes or text.

The subprocess.run with capture_output=True and text=True returns stdout as a string including the newline added by echo command.

❓ assertion

intermediate

Correct assertion for subprocess error

Which assertion correctly checks that a subprocess call failed with exit code 1?

PyTest

import subprocess

def test_fail():
    result = subprocess.run(['false'])
    # Which assertion is correct here?

A assert result.returncode == 1

B assert result.returncode == 0

C assert result.stdout == 'error'

D assert result.stderr == 'failed'

💡 Hint

The 'false' command always exits with code 1 and produces no output.

The 'false' command returns exit code 1, so checking result.returncode == 1 is correct.

🔧 Debug

advanced

Identify the bug in subprocess test

Why does this pytest test fail unexpectedly?

PyTest

import subprocess

def test_ls():
    result = subprocess.run(['ls', '-l'], capture_output=True)
    assert b'README.md' in result.stdout

A The 'ls' command is not available on Windows

B capture_output=True disables stdout

C result.stdout is bytes, so 'README.md' in bytes fails

D The assertion syntax is invalid

💡 Hint

Check the type of result.stdout when capture_output=True is used without text=True.

Without text=True, result.stdout is bytes, so searching a string in bytes fails causing the assertion to fail.

❓ framework

advanced

Best pytest fixture for subprocess cleanup

Which pytest fixture is best to ensure subprocesses are terminated after tests?

A

@pytest.fixture(autouse=True)
def cleanup_subprocess():
    yield
    # terminate subprocesses here

B

@pytest.fixture(scope='session')
def cleanup_subprocess():
    # terminate subprocesses here

C

@pytest.fixture(scope='module')
def cleanup_subprocess():
    yield
    # terminate subprocesses here

D

@pytest.fixture(scope='function')
def cleanup_subprocess():
    yield
    # terminate subprocesses here

💡 Hint

Subprocesses should be cleaned after each test function to avoid interference.

Using scope='function' ensures cleanup runs after each test function, preventing subprocess leaks.

🧠 Conceptual

expert

Why use subprocess.check_output in tests?

What is the main advantage of using subprocess.check_output over subprocess.run in pytest tests?

A check_output raises an exception if the command fails, simplifying error detection

B check_output automatically retries failed commands

C check_output runs commands asynchronously

D check_output captures stderr by default, unlike run

💡 Hint

Think about how exceptions help in test failures.

subprocess.check_output raises CalledProcessError if the command exits with non-zero code, making test failures explicit.