Pytest

Pytest won the Python testing wars, and it wasn't close. Where unittest makes you write Java-style boilerplate in Python, pytest lets tests look like regular functions with regular asserts. The plugin ecosystem (800+) covers everything from async testing to coverage to parallel execution.

If you're writing Python — solo SaaS, API, data pipeline, anything — just use pytest. unittest ships with the standard library and costs nothing to try, but you'll fight its verbosity. Robot Framework targets BDD and non-technical teams, not developers. Nose2 exists but has a fraction of the community. Commercially, there's nothing worth paying for here.

The real power is fixtures. Dependency injection for tests without the ceremony. Parametrize a single test across dozens of inputs in one decorator. The learning curve from "basic tests" to "advanced fixtures" is steeper than expected, but it pays off fast.

The catch: pytest's magic (auto-discovery, fixture injection, conftest.py) can confuse newcomers. When a test fails because of fixture scoping, debugging feels like reading someone else's metaprogramming.