Lumina

Manuals / Python / Ch 7

C · TestingIntermediate45 min read

7. pytest fundamentals

Python · 52 pages source format

Arrange-act-assert. test_ prefix. assert directly. pytest discovers tests/ automatically.

What you'll learn

  • pytest discovery
  • assert style
  • Running pytest
  • Test naming

First tests

test_is_valid_email_true(), test_is_valid_email_false(). No unittest boilerplate.

# tests/test_helpers.py
from src.helpers import is_valid_email

def test_valid_email():
    assert is_valid_email("a@b.com") is True

def test_invalid_email():
    assert is_valid_email("nope") is False

Do this now

5 pytest tests for helpers from earlier chapters. pytest -v.

Clear?

Arrange-act-assert

Setup data, call function, assert outcome. One logical assertion per test when possible.

Do this now

Refactor one vague test into three focused tests.

Clear?

Run and debug failures

pytest shows assertion diffs. pytest -k email runs subset. pytest --lf reruns last failures.

Do this now

Break a test on purpose. Read failure output. Fix it.

Pro tip. Keep tests fast — no network in unit tests. Mock or fixture files instead.

Clear?

Checklist