1. Setting up your environment

Before we write any tests, let's get you a clean project to put them in. This page walks through creating a Python environment with pytest installed. It takes about two minutes, and by the end you'll have everything you need for the rest of the guide.

If you already have a Python project with pytest set up, feel free to skip to the next section.

This guide assumes Python 3.10 or later. Check yours with python --version. If it prints something older, grab a newer one from python.org before continuing.

Create a project folder

Pick a short path on your machine and make a new folder for this project. Something like:

C:\dev\weather-client

On macOS or Linux, ~/dev/weather-client works the same way. Short paths keep your terminal output tight, which matters more than you'd think once you start copying errors and sharing screenshots with teammates.

Open the folder in your editor. The rest of the commands in this guide run from inside it.

Create a virtual environment

Python projects share the global interpreter by default. Every time you run pip install, the package goes into the same bucket, so packages from one project can accidentally affect another, and two projects that need different versions of the same library will fight.

The fix is a virtual environment: a private Python setup scoped to one project, with its own package list. Enter the following command to create one from inside your project folder:

python -m venv .venv

That creates a hidden .venv/ folder containing a private Python interpreter and an empty package list. You only do this once per project.

Activate the environment

Activation tells your terminal “when I type python or pip, use the one inside .venv.” The command is slightly different depending on your OS.

On Windows (PowerShell):

.venv\Scripts\Activate.ps1

On macOS or Linux:

source .venv/bin/activate

You'll know it worked when your prompt picks up a (.venv) prefix:

(.venv) PS C:\dev\weather-client>

Watch for that prefix throughout the rest of the guide. If it's missing, you're running the global Python and things will go wrong in confusing ways. When in doubt, rerun the activate command.

One small gotcha if you're using VS Code: activating the venv in your terminal doesn't automatically tell VS Code's editor about it. For IntelliSense and test discovery to use the right interpreter, hit Ctrl+Shift+P → type Python: Select Interpreter → pick the one inside .venv. The bottom-right status bar will confirm it picked up .venv (3.12.x).

Install pytest

With the environment active, install pytest:

pip install pytest

Pip drops it into .venv/, not your global Python. Verify with:

pytest --version

You should see something like pytest 9.0.3. If you do, you're ready to write tests.

Freeze your dependencies

One more habit worth picking up before we move on. Run:

pip freeze > requirements.txt

That writes a requirements.txt file listing every package in your environment with its exact version. It's how Python projects communicate their dependencies. If a teammate (or future-you on a new machine) wants to recreate your environment, one command does it:

pip install -r requirements.txt

Re-run the freeze any time you install a new package. It's a habit that costs nothing and saves you from “but it works on my machine” arguments later.

Ignore the junk

If you plan to put this project under version control (and you should), create a .gitignore file at the project root with these lines:

.venv/
__pycache__/
*.pyc
.pytest_cache/

Those are the generated folders and bytecode that shouldn't be committed. Every Python project has roughly the same list, and once you've seen it a few times you'll write it from memory.

You're ready

Your project folder should now look something like this:

weather-client/
├── .venv/
├── .gitignore
└── requirements.txt

That's the whole setup. Every other command in this guide assumes you're sitting in this folder with the venv active. Next, we'll write our first test and watch it pass.