Writing and running a file

What you will be able to do

After this lesson you can

• write a program as a file and save it with the right name,
• run it from IDLE and from a terminal,
• move to the right folder with cd, and
• avoid the file name problems that stop most beginners.

A program is a file

A Python program is a plain text file whose name ends in .py, such as hello.py. Python reads the file and follows the lines from top to bottom, exactly as in the code boxes. The only difference is where you write it and how you start it.

Here is a first program for a file.

Where to write it

You can write the file in any editor that makes plain text. These are good for a beginner.

A word processor, such as Word, is not a good choice. It adds formatting to the file and changes the quotation marks.

Running a file in IDLE

1. Open IDLE. (On Windows, find it in the Start menu. On macOS, it is in the folder that the installer opened.)
2. Choose File, then New File.
3. Type print("Hello!").
4. Choose File, then Save, and name it hello.py. Remember the folder where you save it.
5. Choose Run, then Run Module, or press F5. The output appears in the first IDLE window.

Running a file in a terminal

1. Open a terminal, as in the last lesson.
2. Go to the folder of your file with cd. For example, if the file is in Documents, type cd Documents and press Enter.
3. Run the file: python hello.py on Windows (or py hello.py), and python3 hello.py on macOS and Linux.

Some terminal commands help you to find the way.

The terminal shows the folder that you are in. The program can only be found when you are in its folder, or when you write its full name, for example python Documents/hello.py.

Common problems with files

• The file is called hello.py.txt. Windows hides the ending of known file types, and Notepad may add .txt when you save. In File Explorer, turn on File name extensions in the View menu. In Notepad, set Save as type to All files. Run this check for different names:

The terminal always works inside one folder. To run hello.py, first go to the folder that holds it.

Programs that ask: input() in a terminal

On this site, you type the answers of input() in the Input box before you press Run. In a terminal, a program with input() stops and waits. You type the answer and press Enter. The program then goes on, to the next line. It feels like a conversation.

Type your name and your city on two lines of the Input box, and run it. Then save the same program as card.py on your computer and run it from a terminal. Notice that, in the terminal, you type each answer when the question appears.

Common mistakes

• Saving as .txt. The terminal then says that this is not a Python file, or it runs nothing.
• Running from the wrong folder. Use cd and dir or ls.
• Using a word processor. It makes curly quotation marks, and Python rejects them.
• Calling a file random.py, math.py or print.py. It hides the real module with the same name, and gives strange errors. Choose names such as hello.py or my_card.py.
• Forgetting to save before you run. Save the file, and then run.

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 A file name checker (or the program above). Add four names of your own.
2. Build. Open the problem The file name trap, and the problem Write your first file.
3. Stretch. Open the problem Input here and in a terminal. Say what is different, step by step.

Remember these

• A program is a text file that ends in .py.
• IDLE: F5 runs the file. Terminal: python hello.py or python3 hello.py, from the folder of the file.
• cd goes into a folder, dir or ls shows the files.
• Check that the file name does not end in .txt. Do not use the name of a module for your file.

Check yourself

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

1. How do you run hello.py in IDLE?

2. Which command goes into a folder, and which one goes up?

3. Why is hello.py.txt a problem?

4. How does input() work in a terminal?