Installing Python and Running Code

The 24-chapter Python for Beginners series now moves from ideas to your machine. This chapter installs a current Python from python.org, explains the confusing python versus python3 command names, gets you into the REPL, runs a script from a file, and shows the one rule that replaces braces entirely: indentation is the syntax.

Install from python.org

You need the reference implementation, CPython, from python.org/downloads. Any release from 3.12 onwards runs every example in this series — the series uses no third-party packages until chapter 13, so nothing else is required.

1. macOS — download the package installer, run it and accept the defaults. The installer puts python3 on your PATH for you.
2. Windows — download the installer, tick Add python.exe to PATH on the first screen, then choose Install Now.
3. Linux — most distributions already ship Python 3; if not, install it with your package manager, for example sudo apt install python3 on Debian or Ubuntu.

Verify from a fresh terminal — always a fresh terminal, because a new install cannot update a shell that is already running:

python3 --version
python --version

Expected output:

Python 3.13.12
Python 3.13.12

Your first line will show whichever release you downloaded; anything from 3.12 up is correct for this series. If the second command reports that python is not found, that is normal on macOS and Linux and it is the subject of the next section — not a broken install.

python3 versus python

Two commands, one interpreter, and forty years of history. When Python 3 arrived in 2008 it broke compatibility with Python 2, so installers began shipping the new version as python3 and left the bare python name to whatever old version was already present. Python 2 reached end of life in 2021, but the two command names never merged back together.

Rule for this series: on macOS and Linux type python3. On Windows type python or py instead — every command in the series works with that one-word substitution.

Meet the REPL

Running the interpreter with no file opens the REPL: Read, Evaluate, Print, Loop. It reads one statement, evaluates it, prints the result and waits for the next. Start it with python3 (or python on Windows); you will see a banner naming your exact build, then the primary prompt >>>:

>>> 2 + 2
4
>>> name = "Ada"
>>> print("Hello,", name)
Hello, Ada
>>> 7 / 2
3.5
>>> quit()

Three behaviours to notice. The assignment on line three prints nothing — the value went into a name rather than to the printer, so the prompt simply returns (JavaScript consoles answer undefined instead, which is a small mercy you do not get here). Expressions echo their value, always in its literal form. And quit() — or Ctrl+D on macOS and Linux — leaves the session.

Anything unfinished shows the continuation prompt .... A block of code is only released when you press Enter on an empty ... line:

>>> total = 10
>>> if total > 5:
...     print("big")
...
big
>>>

Expected output: the line big, printed once the empty continuation line ends the block. The REPL is the right place to test a single expression; it forgets everything when you close it, so real programs live in files.

Running a script file

Scripts are plain text files with a .py extension. Create hello.py in a folder of your choice:

# hello.py
print("Running my first script")
print("Sum:", 6 * 7)

Open a terminal in that same folder and pass the file to the interpreter:

python3 hello.py

Expected output:

Running my first script
Sum: 42

The working directory matters: python3 hello.py looks for the file relative to where your terminal currently is, so cd into the folder first or pass the path to the file. For a single throwaway expression you can skip the file entirely: python3 -c "print('from a one-liner')" prints from a one-liner without creating anything.

Windows and the py launcher

The Windows installer also provides py, a launcher that knows which interpreters you have installed and can pick between them. It runs the same files as python:

py --version
py hello.py
py -3.12 hello.py

Expected output:

Python 3.13.12
Running my first script
Sum: 42

The first line names whichever version the launcher chose, so it matches whatever you installed. Use the explicit form (py -3.12) once you have more than one release on the machine; otherwise the plain launcher is fine. If you ticked Add python.exe to PATH, python hello.py works as well.

Choose an editor

Any text editor that can save UTF-8 will do, but syntax highlighting and a built-in terminal remove friction. The recommendation for this series is Visual Studio Code with Microsoft’s Python extension: it adds colouring, completion, inline error reporting, a test panel and one-click run — while every example here still runs from a plain terminal.

Install VS Code, open the folder containing your scripts, install the Python extension, then open its integrated terminal and run python3 hello.py exactly as you would in any other terminal. Nothing in this course depends on an IDE feature.

Indentation is the syntax

JavaScript reaches for braces to mark a block; Python reaches for whitespace. The indented lines under if, for, def and class are the block, and PEP 8 asks for four spaces per level, spaces only (never tabs, never a mixture of the two). This file is wrong on line two:

print("one")
  print("two")

Expected output:

  File "hello.py", line 2
    print("two")
IndentationError: unexpected indent

Python prints the full path of your file on the File line; it is shortened to hello.py above. Note what is missing: there is no Traceback header, because a file with broken indentation never gets as far as running — Python parses the whole module before executing a single line of it.

The corrected version, with four spaces under the if and the following line back at the left margin:

total = 10
if total > 5:
    print("big")
print("done")

Expected output:

big
done

That is the whole mechanism. Indentation decides what belongs to the if, and returning to column zero ends it — no closing brace to forget, and no brace to hunt for when something does not line up.

Common mistakes

• Typing python on macOS or Linux — the command may be absent or may still be Python 2; use python3 (or py on Windows).
• Running the script from another folder — can't open file ... [Errno 2] No such file or directory means your terminal is not where the file is; check with pwd and ls.
• Mixing tabs and spaces — an editor that inserts tabs while the example uses spaces produces IndentationError or TabError; set your editor to insert four spaces.
• Pasting a whole block into the REPL — without the final empty line the block never executes; keep multi-line code in files instead.
• Following Python 2 tutorials — print "hi", raw_input() and xrange() are two decades old and simply do not run.

Practice checklist

1. Install CPython from python.org and record the output of python3 --version in your notes.
2. Open the REPL and evaluate five expressions, including one division and one string method, then leave with quit().
3. Create hello.py, run it, change a line, and run it again until the edit-run loop feels automatic.
4. Indent line two of a script wrongly on purpose, read the IndentationError, and fix it by aligning to column zero.
5. Install the VS Code Python extension, open your script folder, and run the file from the integrated terminal.

What's next?

The interpreter runs and your editor is ready — now the language itself begins with names and values: Variables and Data Types.