Using the code boxes on this site

What you will be able to do

After this lesson you can

• run a program in a code box and change it,
• say what each part of a code box does,
• get back the first code when you have changed too much, and
• avoid the small habits that cause most first mistakes.

You do not need to install anything

Every program in this book runs right here, in your browser. There is nothing to install, and you do not need an account to run code. Python works on a computer, a tablet and a phone.

This is how we learn on this site: you read a little, you run a program, you change it, and you see what happens. Reading about programming is like reading about swimming. You learn it when you get into the water.

The parts of a code box

1. The code. Click in it, and you can type or change anything.
2. The buttons. Run starts the program. Reset brings back the first code. Copy copies the code, so that you can paste it somewhere else.
3. The Input box. Some programs ask a question with input(). Type your answers here, one on each line. You will use it in Chapter 4. If you do not see it, press + Input.
4. The output. What the program shows appears here. An error message appears here too.

Your first run

Press Run. The first time, it may take a few seconds, because Python is loading in your browser. After that, it is fast. Then change a word inside the quotation marks, for example first to very first, and press Run again.

What happened? You changed the text, and the output changed. The program does exactly what its lines say, nothing more and nothing less. This is the most important idea in the whole book.

Experiments to try

You cannot break anything in a code box. If the code goes wrong, press Reset. So be brave, and try these, one at a time.

1. Delete the second line. Run it. How many lines does the output have?
2. Write a new line below the others: print("My name is") and then your name in quotation marks. Run it.
3. Change the first line to a comment, by putting # at the start. Run it. What happens to that line? (We study comments in the next lesson.)
4. Press Reset. Is the first code back?

When you want to change the code for real, press Run this code. It opens in a full code box, where you can change anything.

Good habits

• Change things. A change that goes wrong teaches you more than a program that works at the first try.
• Type it yourself. Copying is fine to see how a program works. But typing a program yourself teaches you more, because your fingers learn the shapes.
• Use straight quotation marks. Python wants the plain " or '. Curly quotation marks from a word processor, “ ”, give an error.
• On a phone or a tablet, the signs ( ) [ ] { } " * # are often on the second or third keyboard page.
• If it says that Python failed to load, check your internet connection and refresh the page. The first load needs the internet.
• Run often. Write a line or two, run, look. Do not write twenty lines before you run.

Python on your own computer

Later, you may want to keep your programs as files, or to work without the internet. Chapter 6 shows how to install Python on Windows, macOS and Linux. You can finish Chapters 1 to 5 without it.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Remember these

• Press Run to run the code. Press Reset to go back to the first code.
• Answers for input() go in the Input box, one on each line.
• Change a program, run it, and look at what happened. That is how you learn.
• You do not need to install anything to learn from this book.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which button starts the program?

2. Where do you type the answers for input()?

3. You changed a program and cannot find the mistake. What can you do?

4. Why should you not copy curly quotation marks from a word processor?