pytest-xdist for parallel execution - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pytest-xdist used for?

pytest-xdist is a plugin for pytest that allows tests to run in parallel across multiple CPUs or hosts, speeding up test execution.

beginner

How do you run tests in parallel using pytest-xdist?

Use the command pytest -n NUM where NUM is the number of parallel workers you want to run tests on.

intermediate

What does the -n auto option do in pytest-xdist?

It automatically detects the number of CPU cores on your machine and runs that many parallel test workers.

intermediate

Name one common issue when running tests in parallel with pytest-xdist.

Tests that share or modify global state or resources can cause conflicts or flaky results when run in parallel.

advanced

How can you ensure tests are safe to run in parallel with pytest-xdist?

Design tests to be independent, avoid shared state, use fixtures with proper scope, and isolate external resources like files or databases.

Which command runs tests in parallel using 4 workers with pytest-xdist?

A pytest --workers 4

B pytest --parallel=4

C pytest -p 4

D pytest -n 4

The correct syntax to run tests in parallel with 4 workers is pytest -n 4.

What does pytest-xdist help improve?

A Test case writing automation

B Test code coverage reporting

C Test execution speed by running tests in parallel

D Test result visualization

pytest-xdist improves test execution speed by running tests in parallel.

If tests share global variables, what problem might occur when using pytest-xdist?

A Tests may fail or behave unpredictably

B Tests will run slower

C Tests will skip automatically

D Tests will generate more logs

Sharing global variables can cause conflicts and unpredictable test results when tests run in parallel.

How does pytest-xdist decide the number of workers with -n auto?

A It uses a fixed number 2

B It uses the number of CPU cores available

C It asks the user to input

D It runs tests sequentially

-n auto runs tests using the number of CPU cores detected on the machine.

Which of these is NOT a best practice when using pytest-xdist?

A Run tests that modify the same file simultaneously

B Avoid shared state between tests

C Use fixtures with proper scope

D Isolate external resources

Running tests that modify the same file simultaneously can cause conflicts and should be avoided.

Explain how pytest-xdist speeds up test execution and what you must consider when using it.

Think about how dividing work helps and what problems parallelism can cause.

Describe common issues that can happen when running tests in parallel with pytest-xdist and how to prevent them.

Consider what happens if tests interfere with each other.

Practice

1. What is the main purpose of using pytest-xdist in test execution?

easy

A. To run tests in parallel and reduce total test time

B. To generate detailed HTML reports

C. To automatically fix failing tests

D. To run tests only on a single CPU core

Solution

1. Step 1: Understand pytest-xdist functionality

   pytest-xdist allows running tests simultaneously on multiple CPU cores.

2. Step 2: Identify the benefit of parallel execution

   Running tests in parallel reduces the total time needed to complete all tests.

3. Final Answer:

   To run tests in parallel and reduce total test time -> Option A

4. Quick Check:

   pytest-xdist = parallel test execution [OK]

Hint: Remember: xdist means 'distributed' tests run together [OK]

Common Mistakes:

• Confusing xdist with report generation tools
• Thinking xdist fixes test code automatically
• Assuming xdist runs tests sequentially

2. Which command correctly runs tests in parallel using 4 workers with pytest-xdist?

easy

A. pytest -n 4

B. pytest --workers=4

C. pytest -p 4

D. pytest --parallel 4

Solution

1. Step 1: Recall pytest-xdist syntax for parallelism

   The correct flag to specify number of parallel workers is -n.

2. Step 2: Match the correct command

   pytest -n 4 runs tests with 4 parallel workers.

3. Final Answer:

   pytest -n 4 -> Option A

4. Quick Check:

   -n sets workers = C [OK]

Hint: Use '-n' to set number of parallel workers [OK]

Common Mistakes:

• Using incorrect flags like --workers or --parallel
• Confusing -p (plugin) with -n
• Omitting the number after -n

3. Given this pytest command: pytest -n 3 test_sample.py, what happens during test execution?

medium

A. Tests run in parallel but limited to 1 CPU core

B. Tests in test_sample.py run sequentially on one CPU core

C. Only 3 tests from test_sample.py run sequentially

D. Tests in test_sample.py run in parallel using 3 CPU cores

Solution

1. Step 1: Analyze the command options

   The -n 3 option tells pytest-xdist to use 3 workers for parallel execution.

2. Step 2: Understand effect on test_sample.py

   All tests in test_sample.py are distributed across 3 CPU cores and run simultaneously.

3. Final Answer:

   Tests in test_sample.py run in parallel using 3 CPU cores -> Option D

4. Quick Check:

   -n 3 means 3 parallel workers = A [OK]

Hint: -n sets parallel workers, all tests split among them [OK]

Common Mistakes:

• Thinking only 3 tests run instead of all tests
• Assuming tests run sequentially despite -n
• Confusing number of workers with number of tests

4. You run pytest -n 2 but get an error: ERROR: unknown option: -n. What is the likely cause?

medium

A. You used the wrong flag; it should be --num

B. pytest-xdist plugin is not installed

C. You need to run pytest with sudo

D. The test files are missing

Solution

1. Step 1: Understand the error message

   The error says -n is unknown, meaning pytest does not recognize the option.

2. Step 2: Identify cause related to pytest-xdist

   The -n option is provided by pytest-xdist plugin. If it's not installed, pytest won't recognize it.

3. Final Answer:

   pytest-xdist plugin is not installed -> Option B

4. Quick Check:

   Unknown -n means missing xdist plugin = B [OK]

Hint: Install pytest-xdist to enable -n option [OK]

Common Mistakes:

• Assuming -n flag is built-in pytest option
• Trying to run with sudo unnecessarily
• Ignoring plugin installation step

5. You want to speed up a large test suite using pytest-xdist but notice flaky failures when running with pytest -n auto. What is a good approach to fix this?

hard

A. Disable pytest-xdist and run tests sequentially

B. Increase the number of workers beyond CPU cores

C. Add @pytest.mark.serial to tests that share state to run them sequentially

D. Use pytest -n 1 to force single worker

Solution

1. Step 1: Understand cause of flaky failures

   Tests sharing state or resources can fail when run in parallel due to conflicts.

2. Step 2: Use markers to run conflicting tests sequentially

   Marking such tests with @pytest.mark.serial (or similar) ensures they run one at a time, avoiding conflicts.

3. Step 3: Keep parallelism for other tests

   This approach balances speed and stability by running safe tests in parallel and conflicting ones sequentially.

4. Final Answer:

   Add @pytest.mark.serial to tests that share state to run them sequentially -> Option C

5. Quick Check:

   Mark shared-state tests serial to fix flakes = A [OK]

Hint: Mark shared tests serial to avoid parallel conflicts [OK]

Common Mistakes:

• Blindly increasing workers causing more flakes
• Disabling parallelism losing speed benefits
• Using -n 1 which is just sequential run