Getting Started

This chapter gets a working Python 3 setup onto your machine. You will try code in the interactive interpreter, run a script from a file, give each project its own virtual environment, install packages with pip, and set up an editor that understands your code. By the end you will have a clean, repeatable way to start any new Python project.

Installing Python 3

Python 2 reached end of life years ago, so everything here assumes Python 3. How you install it depends on your platform:

• Windows: use the installer from python.org or the Microsoft Store. If you use the installer, ticking the option to add Python to PATH saves trouble later.
• macOS: use the python.org installer or Homebrew (brew install python).
• Linux: most distributions ship Python already; otherwise install it with your distribution's package manager.

Then check that it works. On Windows, the py launcher is handy because it can pick between installed versions.

python3 --version
# on Windows
py --version

The REPL

Run python3 (or py on Windows) with no arguments and you land in the REPL, a read-eval-print loop. Type an expression after the >>> prompt and the result appears immediately. It is the fastest way to check how a function behaves or to poke at a new library. Use help(len) for built-in documentation, and leave with exit() or Ctrl+D (Ctrl+Z then Enter on Windows).

2 ** 10            # 1024
"python".upper()   # 'PYTHON'
len([1, 2, 3])     # 3
import math
math.sqrt(16)      # 4.0

Running a script

Real programs live in .py files. Save the following as hello.py:

import sys


def main() -> None:
    name = sys.argv[1] if len(sys.argv) > 1 else "world"
    print(f"Hello, {name}!")


if __name__ == "__main__":
    main()

The if __name__ == "__main__": guard means main() runs only when the file is executed directly, not when another module imports it. That small habit keeps your files reusable.

python3 hello.py
python3 hello.py Alice
# -m runs a module by name, here a tiny web server from the standard library
python3 -m http.server 8000

Virtual environments with venv

Different projects need different packages, often at conflicting versions. Installing everything into the system Python quickly turns into a mess. The standard venv module solves this by creating an isolated environment inside your project folder.

# create an environment in a folder called .venv
python3 -m venv .venv

# activate it on macOS or Linux
source .venv/bin/activate

# activate it in Windows PowerShell
.venv\Scripts\Activate.ps1

# leave it
deactivate