Using Python

This book uses Python as its implementation language. Python supports functional, imperative, and object-oriented programming. Plus, it is cross-platform, beginner-friendly, and has many libraries you could use for projects that build on this book. One downside is that Python is quite slow, and for this reason every real web browser is written in C++. For teaching, that isn’t a problem.

Python comes in two major versions: Python 2 and Python 3. All of the examples in this book use Python 3, and if you try to follow along in Python 2 you will get pretty confused. In a few places, I show Python command lines, and when I do I call the Python binary python3. Your system might use something else, but probably doesn’t.

Python Tips

As you’re following along with the text, or implementing exercises, you’ll frequently want to test or debug your web browser. Here are a few tips on doing that.

Test against browsers: Except where explicitly noted, the web browser developed in this book should match the behavior of real web browsers. If you’re unsure what the correct behavior in some situation is, fire up your favorite web browser on an example page. On edge cases, browser behavior is defined more by historical accident and backwards compatibility than by logic. Looking at the real thing is the best way to figure out what to do.

Serving Web Pages: You’ll want to write test pages for your web browser, and you’ll need a web server to serve those pages. Luckily, Python ships with a simple web server. Go to a directory and run:

python3 -m http.server

This will start a web server at the address http://localhost:8000/ serving the contents of index.html in the current directory. You can view other files in the directory by adding them to the URL, like this:

http://localhost:8000/my-test.html

Printable Forms: To use Python’s print function for debugging, you’ll need to define printable forms for the classes you define. Python uses the __repr__ method for this:

class Tag:
    def __repr__(self):
        return "Tag(" + self.tag + ")"

This book won’t define these methods explicitly, but for your own sanity and ease of debugging, implement them whenever you define a class.