Creating Python Programs

After reading the Python tutorial so far, you should have a basic understanding of Python and the REPL. Even though the latter can be very useful, you probably noticed in the last example that the REPL has its limits. It’s nice for quick experiments, but:

• It gets awkward once your code is more than a few lines long
• Fixing a mistake halfway through a longer piece of code is clumsy
• We can’t store our programs and re-use them later on

If we want to store our programs and use a decent editor, we need to simply:

1. create a file,
2. enter the code in that file,
3. save it,
4. … and execute it!

“Wait, what? How!”

No worries, we’ll go over it step by step.

1. Create a Python file

We need to create a so-called plain text file, meaning there’s nothing special about this file. It’s just text. Sounds simple, but it’s not. For example, if you start Word or a similar program, enter some text, and save it, you don’t end up with plain text.

Text processing software like Word will add all kinds of extra codes to define the markup, allow for images to be included, etcetera. It looks a little like HTML, in fact, which is used to create websites.

So what should we use?

If you’re on Windows, try Notepad. It’s a plain text editor that does not add any markup. It’s a horrible way to create Python programs, but we’ll explore better options later on.

If you’re on Linux, you could open a terminal and try a text editor like nano or vim (the last one is hard to use if you haven’t read the manual, though). Many Linux installations include a GUI text editor too, like gedit.

On macOS, you can use TextEdit, but it creates rich text documents by default. Open a new document and choose Format > Make Plain Text before you type anything. If you’d like something nicer, download CotEditor. It’s free and open-source, and will come in handy when you need to have a quick look at any text file in the future.

2. Enter the code

Now it’s time to enter some code. We’ll keep it super simple for now. Just enter something like:

print("Hello world")Code language: Python (python)

On Windows, it should look like this:

3. Save the file

Save the file. The most important thing to look out for is the file extension. Python files end with the .py extension. Call your file helloworld.py. In Notepad, you need to click the dropdown list called ‘Save as type’ and select All files (*.*):

Want to explore what happens when you run saved code? Follow our free lesson on Python files and program runs and try running the same saved program twice to see which values persist.

4. Execute a Python program file

Now that you saved the file, we can execute it. There are two options, and most often you want to go for option one:

1. Open a terminal or command prompt, go to the file’s directory, and run it with the python command (python3 on macOS and Linux)
2. On Windows, double-click the file in File Explorer.

If you try option two, you’ll find out that a window flashes on the screen and you won’t see the output, because the window closes right after the text is printed. So now you know why you want option one! If you create more advanced programs that keep running instead of quitting directly, option two can be an OK way to start a Python script.

Note: I’ll demonstrate using Windows, but this works the same on Linux and macOS. Instead of opening a Command Prompt or Windows Terminal, you open a macOS or Linux terminal, and you type python3 instead of python.

1. Open the start menu and type ‘Terminal’ or ‘Command Prompt.’ It might be named differently if your OS is configured with another language than English. Hint: Create a new profile with English settings. It may be helpful because most examples on the web and in books assume English settings.
2. cd to the directory where you stored your file. In my case, it’s C:\python.land
3. Run the program with the command python helloworld.py. The command py helloworld.py works too.

The output should look like this:

If it doesn’t work

There are two errors almost everyone runs into at this point.

Windows says 'python' is not recognized as an internal or external command, or the Microsoft Store opens. This means Python isn’t installed, or Windows can’t find it. Follow the steps in installing Python. On current Windows versions, that means the Python install manager, which gives you the python and py commands. Open a new terminal window afterward, so it picks up the new commands. On macOS or Linux, the error is command not found, and the fix is the same: install Python first.

Python says it can’t open the file. The message looks something like this:

C:\...\python.exe: can't open file 'C:\\python.land\\helloworld.py': [Errno 2] No such file or directoryCode language: Python (python)

Python works, but your terminal is in a different folder than your file. Type dir on Windows, or ls on macOS and Linux, to list the files in the current folder. If helloworld.py isn’t in that list, use cd to go to the folder where you saved it, and try again. Also check the file name: Notepad sometimes saves it as helloworld.py.txt.

Congrats, You did it. You wrote a Python program, saved it to a file, and executed it like a boss! Don’t celebrate too hard, though. You don’t want to develop Python software using Notepad or other simple text editors. What you really want is a Python IDE: an Integrated Development Environment. Sounds scary, but it’s not!