Pytest

Here’s a comprehensive and structured guide to pytest, covering everything from basics to advanced features, plugins, and best practices — tailored for professional Python developers like yourself.
Author

Benedict Thekkel

🧰 1. What is pytest?

pytest is a powerful, Pythonic testing framework used to write, run, and organize tests with minimal boilerplate.

  • Supports unit, integration, and functional testing
  • Works out-of-the-box with Python’s assert
  • Automatically discovers test files and functions

🚀 2. Getting Started

📦 Installation

pip install pytest

🏃 Running Tests

pytest                # Run all tests
pytest test_file.py   # Run a specific file
pytest -k "name"      # Run tests matching "name"

📄 File & Function Naming Conventions

  • Files: test_*.py or *_test.py
  • Functions: test_*

🧪 3. Writing Basic Tests

def add(x, y):
    return x + y

def test_add():
    assert add(2, 3) == 5

✅ Advantages

  • No need for classes
  • No need for self.assert* syntax

⚙️ 4. Fixtures

Fixtures manage setup and teardown for tests. You can reuse resources like DBs, clients, configs.

Example:

import pytest

@pytest.fixture
def sample_data():
    return {"x": 10, "y": 20}

def test_sum(sample_data):
    assert sample_data["x"] + sample_data["y"] == 30
  • Scope options: function (default), class, module, session

🎯 5. Parametrization

Write a single test function that runs with different inputs:

import pytest

@pytest.mark.parametrize("a,b,result", [
    (1, 1, 2),
    (2, 3, 5),
    (5, 5, 10)
])
def test_add(a, b, result):
    assert a + b == result

🧵 6. Grouping with Classes (Optional)

class TestMath:
    def test_add(self):
        assert 1 + 2 == 3
  • No need to inherit from unittest.TestCase

🔌 7. Plugins & Extensions

Some powerful plugins:

Plugin Purpose
pytest-django Django testing integration
pytest-mock Simplified mocking
pytest-cov Test coverage reporting
pytest-xdist Run tests in parallel
pytest-asyncio Support async tests
pytest-env Set env vars during test runs
pytest-randomly Randomize test order

Install with:

pip install pytest-cov pytest-mock pytest-django

📊 8. Coverage

pytest --cov=my_module
  • Shows % of code covered by tests
  • Use --cov-report=html for an HTML report

🧪 9. Mocks & Patching

Use pytest-mock or unittest.mock:

def test_api_call(mocker):
    mock_request = mocker.patch("requests.get")
    mock_request.return_value.status_code = 200
    ...

🧪 10. Async Testing

Install:

pip install pytest-asyncio

Then:

import pytest
import asyncio

@pytest.mark.asyncio
async def test_async_function():
    result = await some_async_function()
    assert result == "done"

🧪 11. Advanced CLI Options

pytest -x                  # Stop after first failure
pytest -v                  # Verbose output
pytest --tb=short          # Shorter tracebacks
pytest -n auto             # Parallel test execution (requires `xdist`)

📁 12. Test Structure Example

my_project/
├── app/
│   └── logic.py
├── tests/
│   ├── conftest.py       # shared fixtures
│   ├── test_logic.py
│   └── test_api.py

📑 13. conftest.py – Centralized Fixtures

# tests/conftest.py
import pytest

@pytest.fixture
def config():
    return {"ENV": "test"}

Auto-discovered across test files in the same directory tree.

🧪 14. Best Practices

✅ Use descriptive test function names ✅ Keep tests fast, isolated, and repeatable ✅ Avoid hardcoding test data – use fixtures and parametrization ✅ Use pytest-cov to ensure meaningful coverage ✅ Run in CI pipelines (GitHub Actions, GitLab, etc.) ✅ Structure tests like your app structure

🧰 15. CI/CD Integration

# GitHub Actions example
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Setup Python
        uses: actions/setup-python@v4
        with:
          python-version: 3.11
      - run: pip install -r requirements.txt
      - run: pytest --cov=my_app

🧠 Bonus: Debugging Tips

  • pytest --pdb: drop into debugger on failure
  • pytest -s: allow print() output
  • pdb.set_trace() inside your test or app logic

🧠 Example: Test Case with Fixture + Parametrize + Mock

import pytest

@pytest.fixture
def sample_user():
    return {"name": "Ben", "role": "engineer"}

@pytest.mark.parametrize("role", ["admin", "guest"])
def test_user_roles(sample_user, role):
    sample_user["role"] = role
    assert sample_user["role"] in ["admin", "guest"]
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