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Module 08 / Project 03 — Fixtures Advanced

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Focus

  • conftest.py for sharing fixtures across test files
  • Fixture scopes: function, module, session
  • tmp_path for temporary file operations in tests
  • monkeypatch for safely modifying environment variables and attributes

Why this project exists

Real applications read configuration from environment variables, process files on disk, and combine multiple components. Testing these requires setup and teardown that would be tedious to repeat in every test function. Pytest fixtures let you define reusable setup code that runs automatically before each test. conftest.py lets you share fixtures across multiple test files. tmp_path gives you a fresh temporary directory for each test, and monkeypatch lets you safely override environment variables without affecting other tests.

Run

cd projects/modules/08-testing-advanced/03-fixtures-advanced
pytest tests/ -v

Expected output

tests/test_processor.py::test_load_config_reads_env_vars PASSED
tests/test_processor.py::test_load_config_uses_defaults PASSED
tests/test_processor.py::test_process_file_uppercase PASSED
tests/test_processor.py::test_process_file_strips_whitespace PASSED
tests/test_processor.py::test_process_file_missing_file PASSED
tests/test_processor.py::test_save_results_creates_file PASSED
tests/test_processor.py::test_save_results_content PASSED
tests/test_processor.py::test_full_pipeline PASSED

All tests should pass. Each test gets its own temporary directory and environment, so they never interfere with each other.

Alter it

  1. Add a new fixture in conftest.py that creates a temporary directory with three text files of different sizes. Use it in a new test.
  2. Change the sample_config fixture to use scope="module" instead of the default scope="function". Run with -v and notice the setup/teardown happens less often.
  3. Add a monkeypatch fixture that sets APP_DEBUG=true and verify that load_config reads it.

Break it

  1. Remove conftest.py and run the tests. What errors do you get? Why?
  2. Create two fixtures with the same name in different files and see which one wins.
  3. Use monkeypatch.setenv in a test, then check the environment variable in a different test. Is it still set? Why or why not?

Fix it

  1. Put conftest.py back and verify all tests pass again.
  2. Rename one of the duplicate fixtures to avoid the conflict.
  3. Understand why monkeypatch changes are automatically undone after each test (that is the whole point).

Explain it

  1. What is conftest.py and why is it special to pytest?
  2. What is the difference between scope="function" and scope="session" for a fixture?
  3. Why is tmp_path better than creating your own temporary directory manually?
  4. How does monkeypatch differ from directly setting os.environ?

Mastery check

You can move on when you can:

  • Create a conftest.py with shared fixtures from memory.
  • Explain the difference between function, module, and session scope.
  • Use tmp_path to test code that reads and writes files.
  • Use monkeypatch to test code that depends on environment variables.

Next

Project 04 — Property-Based Testing