Hello, Python!

Python is a scripting language, meaning that commands are interpreted at runtime rather than translated into machine code (compiled) beforehand. The simplest way to run a command in Python is making use of the Python command line. Follow these steps:

Open a Command Prompt (Windows) or Terminal (MacOS or Linux) on your computer.
Type python or python3 and press Enter to start the Python interpreter. You should see the Python prompt (>>>).

Terminal/Command Prompt

python3

Python 3.12.5 (v3.12.5:ff3bc82f7c9, Aug  7 2024, 05:32:06) [Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

At the prompt, type a simple Python command like print("Hello, Python!"). Press Enter to execute the command. You should see “Hello, Python!” printed on the screen.

print("Hello, Python!")

Hello, Python!

To exit the Python interpreter, type exit().

Scripts

A Python script is a simple text file containing a set of instructions. Scripts allow us to save instructions to automate repetitive tasks, streamline processes, and perform specific functions without manual intervention. This is called batch processing. Python will read your script, starting at the top and running each line of code until it reaches the bottom.

You can write script files using any text editor. Here we are going to use JupyterLab, so let’s start by going to the text editor panel in JupyterLab and type into it:

print("Hello, Python!")

Then go to File → Save File and you’ll see the small black circle next to the words “untitled.txt” in the tab change to a cross.

We’ve just saved our Python script with the name “untitled.txt” but we should give it a more descriptive name. In the far left pane you will see a list of files. Right click on “untitled.txt” and select Rename. Python files end in the extension “.py” so change its name to “script.py”. You should now see that the name of the text editor tab reads script.py.

To run the script that we’ve just written and saved, we move to the pane on the right with the label “Terminal 1”. This pane gives us access to a command line interface to our computer. This is the same interface that you have in a Command Prompt in Windows or a Terminal on MacOS or Linux. We type commands in here and the computer runs the programmes we ask it to.

Note that

Always give your Python scripts a “.py” extension
In JupyterLab, Python code goes in the text editor window while you run the script from the terminal window

The only command we need to know about here is python (or python3) which we run by typing python in that window, followed by a space, followed by the name of our script file (including the “.py” extension) and then pressing Enter:

Terminal/Command Prompt

python3 script.py

If you do that you should see, printed to the terminal:

script.py

print("Hello, Python!")

Hello, Python!

Congratulations, you’ve just written and executed your very first Python script!

Commenting your script

It is good practice to add comments to your code so it is easy to understand by other programmers or even yourself after a few months. This is particularly important if there’s anything non-obvious in your code. To add a comments you can use a # symbol at any position in your script, what is behind ‘#’ will be ignored by Python when running that line of code.

colour.py

# This script asks for your favourite colour
fav = input("What is your favourite colour?")

# print("Don't print me!") 

print("My favourite colour is", fav) # print my favourite colour