Testing Python

Tests prove features work. Coverage is secondary. E2e tests beat unit tests. Real beats mocked.

Philosophy

• Trustworthiness > coverage. A test that mocks away the tested thing proves nothing.
• 5 good e2e tests > 100 unit tests with heavy mocking.
• Pareto principle. Write the fewest tests that cover 80% of what matters. E2e tests naturally do this.
• Unit tests for pure logic only. Functions that transform data honestly.
• Real over mocked. Real HTTP servers (pytest-httpserver), real tmp dirs, real processes.

Test Planning

When writing new tests, plan before coding: