Setting up testing with pytest and uv

This tutorial guides you through setting up a testing environment using pytest with uv for Python projects. You’ll learn how to create a project with tests, run them efficiently, and implement testing best practices.

Prerequisites

Install uv on your system.

Creating a Project with Tests

Let’s start by creating a sample project with a test directory structure:

$ uv init testing-demo --package
$ cd testing-demo

This creates a Python package project with the following structure:

testing-demo/
├── pyproject.toml
├── README.md
└── src
    └── testing_demo
        └── __init__.py

Adding pytest as a Development Dependency

Add pytest to your project’s development dependencies:

$ uv add --dev pytest

This command:

• Updates your pyproject.toml with pytest as a development dependency
• Creates the project’s lockfile
• Installs pytest in your project’s virtual environment

Creating a Simple Module to Test

Let’s create a simple calculator module to test. Create a new file at src/testing_demo/calculator.py:

def add(a, b):
    return a + b


def subtract(a, b):
    return a - b


def multiply(a, b):
    return a * b


def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

Creating Test Files

Create a tests directory at the root of your project:

$ mkdir tests

Now, create a test file for our calculator module in tests/test_calculator.py:

import pytest
from testing_demo.calculator import add, subtract, multiply, divide


def test_add():
    assert add(1, 2) == 3
    assert add(-1, 1) == 0
    assert add(-1, -1) == -2


def test_subtract():
    assert subtract(3, 2) == 1
    assert subtract(2, 3) == -1
    assert subtract(0, 0) == 0


def test_multiply():
    assert multiply(2, 3) == 6
    assert multiply(-2, 3) == -6
    assert multiply(-2, -3) == 6


def test_divide():
    assert divide(6, 3) == 2
    assert divide(6, -3) == -2
    assert divide(-6, -3) == 2


def test_divide_by_zero():
    with pytest.raises(ValueError):
        divide(5, 0)

Running Tests

Now you can run your tests using uv:

$ uv run pytest

To see more detailed output, use the verbose flag:

$ uv run pytest -v

Adding Test Coverage

Let’s add coverage reporting to see how much of our code is tested:

$ uv add --dev pytest-cov

Now run tests with coverage:

$ uv run pytest --cov=testing_demo

For a more detailed report:

$ uv run pytest --cov=testing_demo --cov-report=term-missing

Configuring Pytest

Now let’s customize the default options when running pytest.

Add the following to your pyproject.toml file:

[tool.pytest.ini_options]
addopts = "--maxfail=1 --cov=testing_demo"

Now re-run pytest on the command line. It will automatically be run with these options set:

• --maxfail=1 stop after the first failure
• --cov=testing_demo generates the coverage report