Test-Driven Development with Python, 3rd Edition

Prerequisites and Assumptions

Here’s an outline of what I’m assuming about you and what you already know, as well as what software you’ll need ready and installed on your computer.

Python 3 and Programming

I’ve tried to write this book with beginners in mind, but if you’re new to programming, I’m assuming that you’ve already learned the basics of Python. So if you haven’t already, do run through a Python beginner’s tutorial or get an introductory book like The Quick Python Book or Think Python, or, just for fun, Invent Your Own Computer Games with Python, all of which are excellent introductions.

If you’re an experienced programmer but new to Python, you should get along just fine. Python is joyously simple to understand.

You should be able to follow this book on Mac, Windows, or Linux. Detailed installation instructions for each OS follow.

Tip

This book was tested against Python 3.13. If you’re on an earlier version, you will find minor differences in the way things look in my command output listings (tracebacks won’t have the ^^^^^^ carets marking error locations for example) so you’re best off upgrading, ideally, if you can.

If you are thinking of using PythonAnywhere rather than a locally installed Python, you should go and take a quick look at [Link to Come] before you get started.

In any case, I expect you to have access to Python, to know how to launch it from a command line, and to know how to edit a Python file and run it. Again, have a look at the three books I recommended previously if you’re in any doubt.

How HTML Works

I’m also assuming you have a basic grasp of how the web works—​what HTML is, what a POST request is, and so on. If you’re not sure about those, you’ll need to find a basic HTML tutorial; there are a few at http://www.webplatform.org/. If you can figure out how to create an HTML page on your PC and look at it in your browser, and understand what a form is and how it might work, then you’re probably OK.

Django

The book uses the Django framework, which is (probably) the most well-established web framework in the Python world. I’ve written the book assuming that the reader has no prior knowledge of Django, but if you’re new to Python and new to web development and new to testing, you may occasionally find that there’s just one too many topics and sets of concepts to try and take on board. If that’s the case, I recommend taking a break from the book, and taking a look at a Django tutorial. DjangoGirls is the best, most beginner-friendly tutorial I know of. The official tutorial is also excellent for more experienced programmers.

JavaScript

There’s a little bit of JavaScript in the second half of the book. If you don’t know JavaScript, don’t worry about it until then, and if you find yourself a little confused, I’ll recommend a couple of guides at that point.

Read on for installation instructions.

Required Software Installations

Aside from Python, you’ll need:

The Firefox web browser

Selenium can actually drive any of the major browsers, but I chose Firefox because it’s the least in hock to corporate interests.

The Git version control system

This is available for any platform, at http://git-scm.com/. On Windows, it comes with the Bash command line, which is needed for the book. See “Windows Notes”.

A virtualenv with Python 3.13, Django 5.1, and Selenium 4 in it

Python’s virtualenv and pip tools now come bundled with Python (they didn’t always used to, so this is a big hooray). Detailed instructions for preparing your virtualenv follow.

Windows Notes

Windows users can sometimes feel a little neglected in the open source world, since MacOS and Linux are so prevalent, making it easy to forget there’s a world outside the Unix paradigm. Backslashes as directory separators? Drive letters? What? Still, it is absolutely possible to follow along with this book on Windows. Here are a few tips:

1. When you install Git for Windows, it will include “Git Bash”. Use this as your main command prompt throughout the book, and you’ll get all the useful GNU command-line tools like ls, touch, and grep, plus forward-slash directory separators.

2. During the Git installation, you’ll get the option to choose the default editor used by Git. Unless you’re already a Vim user (or are desperate to learn), I’d suggest using a more familiar editor, even just Notepad! See Figure P-1.

3. Also in the Git installer, choose “Use Windows’ default console”; otherwise, Python won’t work properly in the Git-Bash window.

4. When you install Python, tick the option that says “Add python.exe to PATH” as in Figure P-2, so that you can easily run Python from the command line.

Figure P-1. Choose a nice default editor for Git

Figure P-2. Add Python to the system path from the installer

The test for all this is that you should be able to go to a Git-Bash command prompt and just run python or pip from any folder.

MacOS Notes

MacOS installations for Python and Git are relatively straightforward:

• Python 3.13 should install without a fuss from its downloadable installer. It will automatically install pip, too.

• Git’s installer should also “just work”.

Similarly to Windows, the test for all this is that you should be able to open a terminal and just run git, python3, or pip from anywhere. If you run into any trouble, the search terms “system path” and “command not found” should provide good troubleshooting resources.

Tip

You might also want to check out Homebrew. It’s a fairly reliable way of installing common Unixy tools on a Mac.¹ Although the normal Python installer is now fine, you may find Homebrew useful in future. It does require you to download all 1.1 GB of Xcode, but that also gives you a C compiler, which is a useful side effect.

Linux Notes

If you’re on Linux, I’m assuming you’re already a glutton for punishment, so you don’t need detailed installation instructions. But in brief, if Python 3.13 isn’t available directly from your package manager,

• On Ubuntu you can install the Deadsnakes PPA. Make sure you apt install python3.13-venv as well as just python3.13 to un-break the default Debian version of Python.

• Alternatively, pyenv is a tool that lets you manage multiple Python versions on the same machine, but it is one more thing to have to learn and remember.

• Alternatively, compiling Python from source is actually surprisingly easy!

However you install it, make sure you can run Python 3.13 from a terminal.

Installing Firefox

Firefox is available as a download for Windows and MacOS from https://www.mozilla.org/firefox/. On Linux, you probably already have it installed, but otherwise your package manager will have it.

Make sure you have the latest version, so that the “geckodriver” browser automation module is available.

Setting Up Your Virtualenv

A Python virtualenv (short for virtual environment) is how you set up your environment for different Python projects. It allows you to use different packages (e.g., different versions of Django, and even different versions of Python) in each project. And because you’re not installing things system-wide, it means you don’t need root permissions.

Let’s create a virtualenv. I’m assuming you’re working in a folder called goat-book, but you can name your work folder whatever you like. Stick to the name “.venv” for the virtualenv, though.

$ cd goat-book
$ py -3.13 -m venv .venv

On Windows, the py executable is a shortcut for different Python versions. On Mac or Linux, we use python3.13:

$ cd goat-book
$ python3.13 -m venv .venv

Activating and Deactivating the Virtualenv

Whenever you’re working through the book, you’ll want to make sure your virtualenv has been “activated”. You can always tell when your virtualenv is active because you’ll see (.venv) in parentheses, in your prompt. But you can also check by running which python to check whether Python is currently the system-installed one, or the virtualenv one.

The command to activate the virtualenv is source .venv/Scripts/activate on Windows and source .venv/bin/activate on Mac/Linux. The command to deactivate is just deactivate.

Try it out like this:

$ source .venv/Scripts/activate
(.venv)$
(.venv)$ which python
/C/Users/harry/goat-book/.venv/Scripts/python
(.venv)$ deactivate
$
$ which python
/c/Users/harry/AppData/Local/Programs/Python/Python312-32/python

$ source .venv/bin/activate
(.venv)$
(.venv)$ which python
/home/myusername/goat-book/.venv/bin/python
(.venv)$ deactivate
$
$ which python
/usr/bin/python

Tip

Always make sure your virtualenv is active when working on the book. Look out for the (.venv) in your prompt, or run which python to check.

Virtualenvs and IDEs

If you’re using an IDE like Pycharm or Visual Studio Code, you should be able to configure them to use the virtualenv as the default Python interpreter for the project.

You should then be able to launch a terminal inside the IDE with the virtualenv already activated.

Activate Not Working on Windows?

If you see an error like this:

bash: .venv/Scripts/activate: No such file or directory

First, double-check you’re in the right folder. Assuming you are, or if you see an error like this:

bash: @echo: command not found
bash: .venv/Scripts/activate.bat: line 4:
      syntax error near unexpected token `(
bash: .venv/Scripts/activate.bat: line 4: `if not defined PROMPT ('

Then you’ve probably run into a old bug where Python wouldn’t install an activate script that was compatible with Git-Bash. Reinstall the latest Python 3, then delete and re-create your virtualenv.

Installing Django and Selenium

We’ll install Django 5.1 and the latest Selenium². Remember to make sure your virtualenv is active first!

(.venv) $ pip install "django<6" "selenium"
Collecting django<6
  Downloading Django-5.1.1-py3-none-any.whl (8.0 MB)
     ---------------------------------------- 8.1/8.1 MB 7.6 MB/s eta 0:00:00
Collecting selenium
  Downloading selenium-4.24.0-py3-none-any.whl (6.5 MB)
     ---------------------------------------- 6.5/6.5 MB 6.3 MB/s eta 0:00:00
Installing collected packages: django, selenium
Successfully installed [...] django-5.1.1 [...] selenium-4.24.0 [...]

Checking it works:

(.venv) $ python -c "from selenium import webdriver; webdriver.Firefox()"

this should pop open a Firefox web browser, which you’ll then need to close.

Tip

If you see an error, you’ll need to debug it before you go further. On Linux/Ubuntu, I ran into this bug which you need to fix by setting an environment variable called TMPDIR.

Some Error Messages You’re Likely to See When You Inevitably Fail to Activate Your Virtualenv

If you’re new to virtualenvs—​or even if you’re not, to be honest—​at some point you’re guaranteed to forget to activate it, and then you’ll be staring at an error message. Happens to me all the time. Here are some of the things to look out for:

ModuleNotFoundError: No module named 'selenium'

Or:

ImportError: No module named django.core.management

As always, look out for that (.venv) in your command prompt, and a quick source .venv/Scripts/activate or source .venv/bin/activate is probably what you need to get it working again.

Here’s a couple more, for good measure:

bash: .venv/Scripts/activate: No such file or directory

This means you’re not currently in the right directory for working on the project. Try a cd goat-book, or similar.

Alternatively, if you’re sure you’re in the right place, you may have run into a bug from an older version of Python, where it wouldn’t install an activate script that was compatible with Git-Bash. Reinstall Python 3, and make sure you have version 3.6.3 or later, and then delete and re-create your virtualenv.

If you see something like this, it’s probably the same issue, you need to upgrade Python:

bash: @echo: command not found
bash: .venv/Scripts/activate.bat: line 4:
      syntax error near unexpected token `(
bash: .venv/Scripts/activate.bat: line 4: `if not defined PROMPT ('

Final one! If you see this:

'source' is not recognized as an internal or external command,
operable program or batch file.

It’s because you’ve launched the default Windows command prompt, cmd, instead of Git-Bash. Close it and open the latter.

On Anaconda

Anaconda is another tool for managing different Python environments. It’s particularly popular on Windows and for scientific computing, where it can be hard to get some of the compiled libraries to install.

In the world of web programming it’s much less necessary, so I recommend you do not use Anaconda for this book.

Apart from anything else I don’t know enough about it to help you debug any problems with it if they occur!

Happy coding!

Note

Did these instructions not work for you? Or have you got better ones? Get in touch: [e-mail]!

¹ I wouldn’t recommend installing Firefox via Homebrew though: brew puts the Firefox binary in a strange location, and it confuses Selenium. You can work around it, but it’s simpler to just install Firefox in the normal way.

² You might be wondering why I’m not mentioning a specific version of Selenium. It’s because Selenium is constantly being updated to keep up with changes in web browsers, and since we can’t really pin our browser to a specific version, we’re best off using the latest Selenium. It was version 4.24 at the time of writing.