github-copilot-instructions pytest.instructions.md

Pytest & Pytest-cov Instructions

Purpose

You WILL use these instructions to write, run, and maintain Python tests using pytest and pytest-cov. These instructions ensure high-quality, maintainable, and well-covered code.

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1. Setup & Installation

You MUST install pytest and pytest-cov in your Python environment:

For example, using pip:

pip install pytest pytest-cov

For example, using poetry:

poetry add --dev pytest pytest-cov

For example, using pipenv:

pipenv install --dev pytest pytest-cov

For example, using uv,

uv add pytest pytest-cov --dev
uv sync

You MUST add them to your requirements.txt or pyproject.toml for project reproducibility.

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2. Test Discovery & Structure

• You MUST place all test files in a tests/ directory or alongside modules as test_*.py or *_test.py.
• You MUST name test functions as test_*.
• You MUST use plain assert statements for test conditions.
• You MUST group related tests in classes prefixed with Test (no __init__ method), and you SHOULD prefer class-based test cases for better organization, extensibility, and shared setup/teardown.
• You MUST keep tests isolated and independent.
• You MUST use fixtures for setup/teardown logic (see below).

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3. Writing Tests

You WILL:

• Prefer class-based test cases for all but the simplest tests, as they:
  • Provide better organization and grouping of related tests.
  • Allow for shared setup/teardown using fixtures.
  • Make it easier to extend and maintain tests as your codebase grows.
• Write one assertion per test unless testing related behaviors.
• Use parameterization (@pytest.mark.parametrize) for repetitive test logic.
• Use fixtures (@pytest.fixture) for reusable setup/teardown.
• Use pytest.raises for exception testing.
• Avoid print statements; use pytest's reporting.
• When using class-based test cases:
  • Test classes MUST be named with a Test prefix (e.g., TestCalculator).
  • Test classes MUST NOT have an __init__ method; use fixtures for setup.
  • You MAY use @pytest.mark.usefixtures or class-level fixtures for setup/teardown.
  • All test methods MUST be named test_*.
  • You MAY use self for shared state within the class, but prefer fixtures for isolation.