Install & Your First Program

To write Python, your computer needs the interpreter - the program that reads .py files and executes them line by line. Installing it trips people up most, usually over a boring reason: the name. Let's install it, prove it's there, and run real code two ways.

📝 Terminology. The Python interpreter is the program named python (or python3) that executes your code. When people say "run it in Python," they mean "hand this file to the interpreter."

Install Python 3

You want Python 3 (3.13 or newer is great; anything 3.10+ is fine for this guide). Pick your operating system:

Windows - Go to python.org/downloads, download the latest Python 3 installer, and run it. On the very first screen, check "Add python.exe to PATH" before clicking Install - that one checkbox prevents the most common beginner headache: the terminal not finding Python afterward.

macOS - macOS ships an old, system-managed Python you shouldn't rely on. Install a fresh one with Homebrew:

$ brew install python

What just happened: Homebrew installed an up-to-date Python 3 and wired up the python3 command. No Homebrew? The python.org installer works just as well.

Linux (Debian/Ubuntu) - Python 3 is usually already present, but install it explicitly to be sure:

$ sudo apt update
$ sudo apt install python3

What just happened: apt (the system package manager) ensured python3 is installed; sudo grants the admin rights that installing system software requires.

Confirm it actually installed

Open a fresh terminal (a new window, so it picks up the updated PATH) and ask Python its version:

$ python3 --version
Python 3.14.0

What just happened: --version makes the interpreter print its version and exit instead of running code. A Python 3.x.x line proves it's installed and reachable - your exact number may differ.

⚠️ python vs python3 - the name confusion that wastes everyone's first afternoon. On macOS and Linux, the command is almost always python3 (plain python may not exist, or may point at an ancient Python 2). On Windows, the installer typically sets up python (and py). If one name gives "command not found," try the other:

$ python --version
Python 3.14.0

This guide writes python3 throughout. On Windows, type python (or py) wherever you see python3 - same interpreter, different spelling.

⚠️ "command not found" / "not recognized." If neither name works, the interpreter is installed but your terminal can't find it - a PATH problem. On Windows, re-run the installer, check "Add python.exe to PATH" (or choose "Modify" and enable it), then open a new terminal - PATH changes only apply to terminals opened afterward.

The REPL - a place to try one line at a time

Run python3 with no file name to get the REPL - an interactive prompt where you type one line, press Enter, and immediately see the result. It's the fastest way to test an idea.

$ python3
Python 3.14.0 (main, Oct  7 2025, 12:00:00) [Clang 17.0.0] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 2 + 2
4
>>> print("hi from the REPL")
hi from the REPL
>>>

What just happened: >>> is the REPL's prompt. You typed 2 + 2; the interpreter evaluated it and printed 4 back - no print needed, the REPL shows the value of whatever you type. It's a scratchpad: try a snippet, see what it does, throw it away.

📝 Terminology. REPL stands for Read-Eval-Print Loop: it reads your line, evaluates it, prints the result, then loops back for the next one.

To leave the REPL and return to your normal terminal, type exit() and press Enter (or press Ctrl-D on macOS/Linux, Ctrl-Z then Enter on Windows):

>>> exit()
$

What just happened: exit() told the interpreter to quit; you're back at your shell's $ prompt. The REPL forgets everything on close - exactly why real programs live in files.

Your first program in a file

A REPL is great for experiments, but real programs are saved files you can run again. Create a file called hello.py in any folder, with one line in it:

print("Hello, Python!")

What just happened: The same print instruction from the REPL, now saved to disk. print displays whatever's in the parentheses - here, the text in quotes. (New to print and "argument"? Programming From Zero, Phase 1 walks through them slowly.)

Now run the file by handing it to the interpreter:

$ python3 hello.py
Hello, Python!

What just happened: python3 hello.py read the file and ran its instructions top to bottom, hit the print line, and showed your text - a complete Python program, written, saved, and run.

⚠️ can't open file 'hello.py'. Seeing python3: can't open file 'hello.py': [Errno 2] No such file or directory means your terminal isn't in the folder where you saved the file. cd into that folder (e.g. cd Desktop) and run the command again - the interpreter looks for files relative to where your terminal is "standing."

💡 Key point. Two ways to run Python, two jobs. The REPL (python3 alone) tries one line at a time and throws it away. A file (python3 yourfile.py) is for real, repeatable programs. You'll live in files, keeping the REPL open on the side for quick tests.

Recap

1. Install Python 3 - python.org on Windows (check Add to PATH), Homebrew on macOS, apt on Linux.
2. python3 --version confirms it's installed and reachable. On Windows, use python or py.
3. The command name is the #1 gotcha: python3 on macOS/Linux, usually python on Windows. "Not found" means a PATH problem - reinstall with PATH enabled, open a fresh terminal.
4. The REPL (python3 with no file) runs one line at a time and prints results; exit() leaves it.
5. Save real code in a .py file and run it with python3 yourfile.py.

Next: values, the types they come in, and Python's use of indentation to structure code.

← Guide overview · Phase 2: Syntax, Values & Types →