Setting Up Python

You need two things to program in this course: Python 3 itself, and somewhere to type code. The lab machines already have both. This page is for getting the same setup at home, and for understanding what the pieces are — because “it works in class but not on my laptop” is a problem you can now diagnose rather than suffer.

Check what you already have

Open a terminal — Terminal on macOS or Linux, PowerShell or Command Prompt on Windows — and type:

python3 --version

If Python is installed you get a line naming the version, such as Python 3.12.4. Anything from 3.10 upward is fine for everything we do. On Windows the command is often just python --version; try both before concluding it is missing. A “command not found” or “not recognised” message means either Python is not installed or the terminal does not know where to find it.

Install it if you need to

Python comes from python.org, whose Downloads page offers the right installer for whatever machine you are on. Take the current version and accept the default options, with one exception worth watching for: on Windows the installer offers a checkbox about adding Python to your PATH. Tick it. That checkbox is what makes the terminal command above work, and skipping it is the single most common cause of “I installed it and nothing happened.”

The REPL: a place to try one line

Typing python3 on its own — no file name — starts an interactive session. The >>> prompt means Python is waiting; it reads what you type, evaluates it, prints the result, and loops, which is why it is called a REPL:

>>> 2 + 2
4
>>> name = "Ada"
>>> print(f"Hello, {name}")
Hello, Ada
>>> exit()

The REPL is perfect for settling an argument about what one line does, and useless for writing a program — nothing you type there is saved. Programs live in files whose names end in .py.

An editor for the files

Any program that saves plain text can write Python, but a code editor makes life much easier: it colours the code, helps with indentation, and runs the file without a trip to the terminal. Two common choices are Thonny, which is built for people learning and ships with Python included, and Visual Studio Code, which is what a great many professionals use. Either is fine, and so is anything else your family already has installed. Use whatever is on the lab machines when you are in class; the code is identical everywhere.

To run a saved file from a terminal, move to the folder it is in and type the file name after the command:

python3 hello.py

First program checklist

Type this — do not paste it — into a file called hello.py, then run it:

name = input("What is your name? ")
hours = float(input("How many hours did you sleep last night? "))
print(f"Thanks, {name} — that is {hours * 60:.0f} minutes.")

• It asks both questions, waits for you, and answers with a number
• Change the wording, run again, and see your change appear
• Answer the second question with eight instead of 8, and read the crash: Python reports ValueError: could not convert string to float: 'eight', which is it telling you exactly what happened — not a punishment
• Add a third question and a third line of output, by yourself

If it crashes for a different reason: capital letters matter (print, not Print), and every opening quote and bracket needs its closing twin. Read the message before retyping anything — Reading an Error Message shows how — and know that crashing your first program is the traditional welcome to programming. You are now set up for Your First Python Program.