Pytest Setup

Skill: Test Framework Setup (pytest)

When to Use This Skill

This skill activates when you need to:

• Set up pytest from scratch in a Python project
• Add coverage reporting to existing tests
• Configure watch mode for test-driven development
• Fix broken pytest configurations

Prerequisites

Check if testing is already configured:

• Look for {{test_dirs}} (e.g., tests/, test/, or __tests__/)
• Check for pytest in requirements.txt, pyproject.toml, or Pipfile
• Verify {{test_command}} exists in project configuration

If already configured: This skill provides configuration improvements and troubleshooting.

Step-by-Step Workflow

Step 1: Install pytest and Extensions

If using pip (requirements.txt):

pip install pytest pytest-cov pytest-watch

Add to requirements.txt or requirements-dev.txt:

pytest>=7.4.0
pytest-cov>=4.1.0
pytest-watch>=4.2.0

If using Poetry:

poetry add --group dev pytest pytest-cov pytest-watch

If using Pipenv:

pipenv install --dev pytest pytest-cov pytest-watch

If using conda:

conda install pytest pytest-cov
pip install pytest-watch

Step 2: Create Test Directory Structure

# Create main test directory
mkdir -p tests

# Create __init__.py to make it a package
touch tests/__init__.py

# Create conftest.py for shared fixtures
touch tests/conftest.py

# Create your first test file
touch tests/test_example.py

Add a sample test to tests/test_example.py:

def test_basic_example():
    """Basic example test to verify pytest is working."""
    assert 1 + 1 == 2
    
def test_string_operations():
    """Test string operations."""
    assert "hello".upper() == "HELLO"

Step 3: Add pytest Configuration

Option A: pytest.ini (Recommended)

Create pytest.ini in project root:

[pytest]
testpaths = tests
python_files = test_*.py *_test.py
python_classes = Test*
python_functions = test_*
addopts = 
    -v
    --strict-markers
    --cov=src
    --cov-report=html
    --cov-report=term-missing
    --cov-report=xml
markers =
    slow: marks tests as slow (deselect with '-m "not slow"')
    integration: marks tests as integration tests

Option B: pyproject.toml

If using pyproject.toml, add:

[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py", "*_test.py"]
python_classes = ["Test*"]
python_functions = ["test_*"]
addopts = [
    "-v",
    "--strict-markers",
    "--cov=src",
    "--cov-report=html",
    "--cov-report=term-missing",
]
markers = [
    "slow: marks tests as slow",
    "integration: marks tests as integration tests",
]

Note: Update --cov=src to match your source directory:

• --cov=src if source is in src/
• --cov=app if source is in app/
• --cov=. for current directory
• --cov=mypackage for package name

Step 4: Create conftest.py with Shared Fixtures

Add to tests/conftest.py:

"""
Shared pytest fixtures and configuration.
"""
import pytest


@pytest.fixture
def sample_data():
    """Provide sample data for tests."""
    return {
        "name": "Test User",
        "email": "test@example.com",
        "age": 25
    }


@pytest.fixture
def temp_directory(tmp_path):
    """Provide a temporary directory for tests."""
    test_dir = tmp_path / "test_workspace"
    test_dir.mkdir()
    return test_dir

Step 5: Run Tests

Basic test run:

pytest

Verbose output:

pytest -v

With coverage:

pytest --cov=src --cov-report=html

Run specific test file:

pytest tests/test_example.py

Run specific test:

pytest tests/test_example.py::test_basic_example

Run tests matching pattern:

pytest -k "test_string"

Skip slow tests:

pytest -m "not slow"

Step 6: Set Up Watch Mode (Optional)

Run pytest-watch for continuous testing:

# Watch for changes and re-run tests
ptw

# Watch with verbose output
ptw -- -v

# Watch specific directory
ptw tests/

Alternative: pytest-xdist for parallel execution:

pip install pytest-xdist
pytest -n auto  # Auto-detect CPU count
pytest -n 4     # Use 4 workers

Common Issues and Solutions

Issue: Tests not discovered

Symptoms: pytest finds 0 tests or can't find test files

Solutions:

1. Verify test file naming: test_*.py or *_test.py
2. Check test function naming: must start with test_
3. Ensure tests/ directory has __init__.py
4. Check testpaths in pytest configuration
5. Run with verbose discovery: pytest --collect-only -v

Issue: Coverage not showing or incorrect

Symptoms: Coverage report shows 0% or doesn't include your code

Solutions:

1. Verify --cov= points to correct source directory
2. Check that source code is in the specified directory
3. Ensure __init__.py exists in source directories
4. Run: pytest --cov=. --cov-report=term to see detailed report
5. Check .coveragerc or pyproject.toml coverage config

Issue: Import errors in tests

Symptoms: ModuleNotFoundError when running tests

Solutions:

1. Install package in development mode: pip install -e .
2. Add project root to PYTHONPATH: export PYTHONPATH="${PYTHONPATH}:${PWD}"
3. Create conftest.py with path modifications:

import sys
from pathlib import Path
sys.path.insert(0, str(Path(__file__).parent.parent / "src"))

4. Ensure setup.py or pyproject.toml is configured correctly

Issue: Fixtures not found

Symptoms: fixture 'name' not found

Solutions:

1. Ensure conftest.py exists in tests/ directory
2. Check fixture name spelling matches exactly
3. Verify fixture is defined with @pytest.fixture decorator
4. Check fixture scope if using scoped fixtures

Issue: Slow test execution

Solutions:

1. Use pytest-xdist for parallel execution: pytest -n auto
2. Mark slow tests: @pytest.mark.slow and skip: pytest -m "not slow"
3. Use fixtures to share expensive setup
4. Cache database/API responses in tests
5. Profile tests: pytest --durations=10 to see slowest tests

Success Criteria

• ✅ pytest runs successfully with pytest -v
• ✅ Coverage report generated in htmlcov/ directory
• ✅ Tests are discovered automatically
• ✅ Sample test passes
• ✅ Coverage percentage displayed in terminal
• ✅ Watch mode works (if installed)

Additional Configuration

Integration with VS Code

Add to .vscode/settings.json:

{
    "python.testing.pytestEnabled": true,
    "python.testing.unittestEnabled": false,
    "python.testing.pytestArgs": [
        "tests",
        "-v"
    ]
}

Integration with CI/CD

GitHub Actions example:

- name: Run tests with pytest
  run: |
    pip install pytest pytest-cov
    pytest --cov=src --cov-report=xml
    
- name: Upload coverage to Codecov
  uses: codecov/codecov-action@v3
  with:
    file: ./coverage.xml

Pre-commit Hook

Add to .pre-commit-config.yaml:

- repo: local
  hooks:
    - id: pytest-check
      name: pytest
      entry: pytest
      language: system
      pass_filenames: false
      always_run: true

Related Skills

• run-tests - Commands to execute tests in configured projects
• debug-test-failures - Debugging failing tests
• code-formatting - Code formatting with black/ruff

Documentation References

• pytest Documentation
• pytest-cov Documentation
• pytest-watch GitHub

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