# Python Tests

> Python tests for NeMo Fabric; use this when writing tests

- Skill: `nvidia/python-tests` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nvidia/python-tests`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nvidia/python-tests/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- License: Apache-2.0
- Author: NVIDIA (https://skillmd.com/u/nvidia)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nvidia/python-tests

---



# Python Test Style

- Pytest is used to run tests.
- Do not add `@pytest.mark.asyncio` to any test. Async tests are automatically detected and run by the async runner; the decorator is unnecessary clutter.
- Do not add a `-> None` return type annotation to test functions. This is not a common convention in pytest and adds unnecessary verbosity.
- When mocking a class, do not define a new class. Use `unittest.mock.MagicMock` or `unittest.mock.AsyncMock`, with the `spec` constructor argument when necessary.
- The name of the mocked class should be prefixed with `mock`, not `fake`.
- Prefer pytest fixtures over helper methods.
- Do not repeat fixtures, if a fixture is needed in multiple test files, place it in a `conftest.py` file.
- When creating a fixture follow this pattern:
  ```python
  @pytest.fixture(name="<fixture_name>"[, scope="<scope>"])
  def <fixture_name>_fixture() -> <return_type>:
      ...
  ```
  Only specify the scope argument when the value is something other than "function".
- Prefer `pytest.mark.parametrize` over creating individual tests for
  different input types.
- If a fixture is needed for a test, but either does not return a value or the value is not used in the test, use the `@pytest.mark.usefixtures` decorator.
- `tests/conftest.py` contains a `restore_environ_fixture` fixture that restores the environment variables to their original state after each test, it is defined with `autouse=True` so it is automatically applied to all tests. If you need to modify the environment variables in a test, do so using `os.environ` and the fixture will restore them after the test completes. There is no need to use `monkeypatch.setenv` to modify environment variables in tests.
- Avoid defensive programming in tests. If a test fails, it should fail loudly and clearly, rather than silently passing due to defensive checks. For example
  ```python
  data = results["data"]
  ```
  is preferred over
  ```python
  data = results.get("data")
  ```
  Simply allow the resulting KeyError to be raised if the "data" key is not present in the results dictionary, as this will provide a clear indication of what went wrong in the test.

## Common Commands

```bash
# Focused test loop
uv run pytest -k "<pattern>"

# Run all tests
uv run pytest
```

## Do Not Write Tests For

- Documentation.
- Test helper code under `tests/_utils/`.
- Package metadata or wheel installation behavior.

## References

- `pyproject.toml`
- `tests/conftest.py`