Installing Python

What you will be able to do

After this lesson you can

• install Python on Windows, macOS or Linux,
• explain what the PATH box is for,
• open a terminal and check the version, and
• know what to do when the terminal cannot find Python.

The five steps

The official place to get Python is python.org. Open the Downloads page. It offers the current version for your system. Do not use other websites for the installer.

Windows

1. Open python.org, go to Downloads, and press the yellow button to download the installer.
2. Open the file that you downloaded.
3. On the first screen, tick the box Add python.exe to PATH. This is the most important step. Then press Install Now.

4. Wait until it says that the setup was successful, and press Close.

What is the PATH? Your computer has a list of folders where it looks for programs when you type a command. This list is called the PATH. If the folder of Python is on the list, the terminal finds Python when you type python. If it is not, the terminal says "not recognised". The box Add python.exe to PATH puts Python on the list. It is the step that beginners forget most often.

Windows also installs the Python launcher, a small program called py. It finds your Python for you. That is why py --version often works even when python --version does not.

macOS

1. Open python.org, go to Downloads and download the macOS installer.
2. Open the downloaded file and press Continue through the steps. Press Agree and Install. Type your computer password if it asks.
3. When it is done, the installer opens a folder that holds IDLE, the small editor that comes with Python.

On macOS, the command is python3, with the number. Older versions of macOS have an old Python 2 under the plain name python, and it is not the one that you want.

Linux

Most Linux systems have Python 3 already. Open a terminal and check (see below). If it is missing, install it with your package manager. On Ubuntu and Debian:

sudo apt update
sudo apt install python3 idle3

The first line refreshes the list of programs. The second one installs Python 3 and IDLE. The word sudo asks for the administrator's password. On other kinds of Linux the package manager has another name, such as dnf or pacman, but the idea is the same.

The download is the only step that needs the internet. After that, Python works offline.

Check that it worked

Open a terminal.

• Windows: press the Start button, type cmd, and press Enter. This opens Command Prompt.
• macOS: press Command and Space, type Terminal, and press Enter.
• Linux: press Ctrl, Alt and T, or look for Terminal in the menu.

Type the version command and press Enter.

If you see Python 3. followed by numbers, it worked. The numbers can be different from the picture. The first one must be 3.

If the terminal says that it cannot find Python

• On Windows, try python --version as well as py --version.
• On macOS and Linux, the command is python3, not python.
• On Windows, if you forgot the PATH box, run the installer again, choose Modify, and tick it, or install again with the box ticked.
• Close the terminal and open a new one. An old terminal does not know about the new installation. This single step solves many problems.
• On Windows, if typing python opens the Microsoft Store, the PATH is not set, or a shortcut of the Store is in the way. Use py, or reinstall from python.org.

A program inside the terminal

Python can also be used directly in the terminal. Type python (or py, or python3) and press Enter. You see >>>. Now type 2 + 3 and press Enter. It answers 5. This is the interactive mode. It is a good place to try one small thing. To leave it, type exit() and press Enter. Later in the book, you will write whole programs as files, which is the next lesson.

Common mistakes

• Not ticking the PATH box on Windows. The terminal then cannot find Python.
• Typing the version command into Python, after the >>>. The version command goes in the terminal before you start Python.
• Using an old terminal window after the installation.
• Typing python on macOS or Linux. Use python3.
• Downloading a very old version from a site that is not python.org.

Practice

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

Then the labs and the problems:

1. Warm-up. Open the lab Choose the command. It asks you for the version command for each system.
2. Build. Open the problem Put the steps in order, and the problem Which command?
3. Stretch. Open the problem The terminal cannot find Python. Write a list of the things that you would try, in order.

Remember these

• Download only from python.org.
• On Windows, tick Add python.exe to PATH on the first screen.
• Check with py --version (Windows) or python3 --version (macOS and Linux).
• A new installation needs a new terminal window.

Check yourself

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

1. Which website gives the official installer?

2. Which box must you tick on Windows, and what does it do?

3. Which command checks the version on macOS?

4. You installed Python, but the terminal does not know it. What is the first thing to try?