About Testing Your Code in Python (Part 2) - Writing Your First Tests

In this second part of our testing series, we’ll dive into writing actual tests using pytest.

Setting Up Your Test Environment

First, let’s install pytest:

pip install pytest

You can verify the installation with pytest --version.

Writing Your First Test

A pytest test is simply a Python function that starts with test_:

def test_addition():
    """Test that addition works correctly."""
    assert 1 + 1 == 2

def test_subtraction():
    """Test that subtraction works correctly."""
    assert 5 - 3 == 2

class TestCalculator:
    """Group related tests in a class."""

    def test_multiply(self):
        result = 3 * 4
        assert result == 12

    def test_divide(self):
        result = 10 / 2
        assert result == 5.0

Using Fixtures

Fixtures help you set up test data and resources:

import pytest

@pytest.fixture
def sample_data():
    """Provide sample data for tests."""
    return {
        "name": "John Doe",
        "age": 30,
        "email": "[email protected]"
    }

def test_user_name(sample_data):
    assert sample_data["name"] == "John Doe"

def test_user_age(sample_data):
    assert sample_data["age"] >= 18

Running Tests

To run your tests, simply execute:

pytest

For more verbose output:

pytest -v --tb=short

pytest will automatically discover and run all test functions in files matching test_*.py or *_test.py.

Configuration Example

You can configure pytest using a pyproject.toml file:

[tool.pytest.ini_options]
minversion = "7.0"
addopts = "-ra -q"
testpaths = [
    "tests",
]
pythonpath = [
    "src"
]

Best Practices

1. Keep tests simple and focused - Each test should verify one specific behavior
2. Use descriptive names - Test names should clearly describe what’s being tested
3. Arrange, Act, Assert - Structure your tests in these three phases

Next Steps

In the next part, we’ll explore fixtures and how they can help reduce code duplication in your tests.