pytest

Using pytest in production CI/CD pipelines for years, it consistently delivers on performance and reliability. Test collection is fast even with thousands of tests, parallel execution via pytest-xdist scales linearly, and memory usage stays predictable. The fixture system with scope control (function/class/module/session) gives precise resource management - critical when dealing with database connections or external services that need proper cleanup.

The CLI flexibility is outstanding for different environments: `-x` for fail-fast in development, `--tb=short` for CI logs, `-v` levels for debugging, and `--durations` to find slow tests. Logging integration captures stdout/stderr cleanly without interfering with test output. The plugin system is stable - we've upgraded from pytest 4.x through 9.x with minimal breaking changes, mostly around deprecated features with clear migration warnings.

Error reporting is exceptional. Stack traces are clean and actionable, assertion introspection shows actual vs expected values without boilerplate, and the `-vv` flag provides variable dumps. Configuration via pytest.ini or pyproject.toml is straightforward, and marks/parametrize let you control test execution without code duplication.

Coming from unittest, pytest was refreshingly simple to pick up. The ability to write tests with plain assert statements instead of self.assertEqual() methods immediately reduced cognitive load. The documentation is excellent, with the 'Getting Started' guide having you writing real tests in minutes. Fixtures took a bit longer to grasp conceptually, but once it clicked, the dependency injection pattern felt natural.

Error messages are where pytest truly shines - when an assertion fails, you get detailed output showing exactly what values were compared, with intelligent introspection that saved me countless debugging hours. The parametrize decorator makes data-driven testing straightforward, though the syntax can look cluttered with complex datasets. Stack Overflow coverage is extensive; I've never had a pytest question go unanswered.

Day-to-day usage is smooth. Plugin discovery is automatic, fixtures scope naturally, and the command-line interface is intuitive with helpful filtering options. The learning curve is gentle - you can be productive immediately with basic asserts, then gradually adopt fixtures, marks, and plugins as needs arise.

Coming from unittest, pytest feels like a breath of fresh air. The learning curve is surprisingly gentle - you can start writing tests with plain assert statements instead of memorizing assertThis() and assertThat() methods. The auto-discovery just works when you follow the test_*.py convention, and fixtures are intuitive once you grasp the dependency injection pattern.

Error messages are exceptional. When assertions fail, pytest shows you the actual values being compared with intelligent diffs. The traceback filtering helps you focus on your code rather than framework internals. Debugging is straightforward with --pdb flag dropping you into the debugger exactly where tests fail.

The plugin ecosystem is massive and well-documented. Common needs like coverage (pytest-cov), mocking enhancements (pytest-mock), and parallel execution (pytest-xdist) integrate seamlessly. The official docs have a solid "how-to" section covering real scenarios, and Stack Overflow has answers for virtually every edge case. When I've filed GitHub issues, maintainers respond thoughtfully and quickly.