Setting Up Without the Ceremony

You need exactly one thing to start: a working Python interpreter you can reach from a terminal. You do not need an editor with plugins, a virtual environment, or a project folder yet — those matter later, once you have more than a few lines to organise.

Type python3, specifically — not the bare python. Python 2 reached its end of life in January 2020, and on some older Linux and macOS installs, the unqualified python command still quietly points at the version nobody should be writing new code against. Everything in this track, and almost everything written since 2020, targets Python 3.

Typing python3 (or py on Windows) with nothing after it drops you into the interactive interpreter, usually called the REPL — read, evaluate, print, loop. Type an expression, press return, and it is evaluated immediately.

$ >>> 2 + 2$ 4$ >>> "hello".upper()$ 'HELLO'

Everything after the >>> was typed by you. Everything on the line below it is Python printing the result back, unprompted — the REPL shows you the value of every expression you enter, which is why it is the fastest way to check a small idea.

Not every line prints something. An assignment runs silently, because storing a value and evaluating one are different actions — Python only ever prints the second kind. Name the variable on its own line afterwards, and it prints exactly like anything else would.

$ >>> x = 5$ >>> x$ 5

Leave the REPL with exit(), or Ctrl+D on macOS and Linux, Ctrl+Z then Enter on Windows.

The REPL is for trying something out. A .py file is for keeping it. Save a few lines in a file named hello.py, and run the file itself rather than typing its contents by hand:

print("Hello, world!")

Try itYou can also run it right here. Change the message, add a second print line, and run it again.

$ python3 hello.py$ Hello, world!

Nothing you type into the REPL is saved anywhere once you close it. Every program in the rest of this track assumes you are working in a file, not the REPL, for exactly that reason.

Run the command from the same folder the file is saved in, or point at it with a path — python3 scripts/hello.py works one directory up. Get the path wrong and Python says so plainly: can't open file 'hello.py': No such file or directory is a path problem, not a Python problem, and no amount of fixing your code will touch it.

Every package you install lands in one place by default: the interpreter itself. That is fine until a second project needs a different, incompatible version of the same package — which happens more often than it sounds like it should. A virtual environment is a private copy of the interpreter and its installed packages, scoped to a single project folder, so two projects never fight over the same shared copy of anything.

1. 1

   Create one, inside your project folder

   One command makes a folder holding a private copy of Python and pip, separate from the system installation.

2. 2

   Activate it

   The command differs by platform. Once active, every python3 and every pip install you run uses this private copy, not the system one.

3. 3

   Install into it

   From here on, pip install only ever affects this one project.

$ python3 -m venv .venv$ source .venv/bin/activate# the prompt now starts with (.venv)$ pip install requests

Leave it with deactivate, typed on its own — nothing about a virtual environment is permanent. It is a folder you can delete and recreate at any time, which is exactly why the advice to make one is so unconditional.