When it does not work, and what to do next

What you will be able to do

After this lesson you can

• find the cause of the most common problems quickly,
• read an error message from a file in a terminal,
• install a package with pip, and
• choose what to do next.

The six problems that beginners meet

Almost every problem in the first week is one of these six. Each one has a very short check.

Errors in a file

An error in a file looks like the one in the code box on this site, with one difference: the file name is shown.

Packages and pip

Python comes with many tools, such as math and random. For other jobs, people have written packages: ready-made programs that you can add. The tool that installs them is pip. You type its commands in a terminal, and not in a code box. The usual form is python -m pip install name (use py -m pip on Windows if you prefer, and python3 -m pip on macOS and Linux).

After you install a package, you use it in your file with import numpy. A package is installed once, and used in any program afterwards.

Make a plan that you can keep. The next chapters each take a few hours.

What to do next

You now have Python on your computer, and you can run a file. Here is a sensible path.

1. Go on with this book, in the code boxes. Chapter 7 is about decisions with if.
2. Run the same programs as files. Copy a program from a lesson into a file, save it with a good name, and run it from the terminal. It is the best way to learn the tools.
3. Keep a folder called python with a subfolder for each chapter. A good name makes a program easy to find next month.
4. Try a different editor after a few weeks. Thonny shows the variables. Visual Studio Code can run, check and help you write.
5. Do not hurry to install many packages. Learn the basic language first.

Common mistakes

• Typing pip commands in the Python window (after >>>). They go in the terminal.
• Installing a package for the wrong Python, when two Pythons are on the computer. The form python -m pip uses the Python that you mean.
• Giving up at the first error. Every programmer has these problems. Use the table.
• Mixing the two worlds. On this site, answers go in the Input box. In a terminal, you type them when asked.

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 Which Python is this?, and read the version again.
2. Build. Open the problem The terminal cannot find Python, and use the table of this lesson to answer.
3. Stretch. Open the lab Choose the command, and add the three pip commands to your notes.

Remember these

• The six common problems: PATH, python3, the wrong folder, .txt names, Python 2 habits, and names of modules.
• An error in a file is read from the last line, with the line number.
• python -m pip install name adds a package. It is typed in a terminal, and used with import.
• Keep going with the book. The code boxes and your own computer are both fine.

Check yourself

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

1. On Windows, the terminal says that python is not recognised. What do you try first?

2. Why must you not call your own file random.py?

3. Where do you type a pip command?