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

Once activated, python and pip point into the environment and your prompt shows (.venv). The folder can always be recreated, so keep it out of version control by listing it in .gitignore. If you are ever unsure which interpreter is running, python -c "import sys; print(sys.executable)" prints its path; when that path is inside .venv, the environment is active.

Installing packages with pip

pip downloads packages from the Python Package Index (PyPI). Always install into an activated virtual environment, and prefer python -m pip so there is no doubt about which interpreter receives the package.

python -m pip install requests
python -m pip install --upgrade requests
python -m pip list
python -m pip uninstall requests

Setting up your editor

Any text editor will do, but a few features make a big difference:

1. Language support: in VS Code install Microsoft's Python extension; if you prefer JetBrains tools, PyCharm works out of the box.
2. Interpreter selection: point the editor at the Python inside your project's .venv, so completion and error highlighting know about the packages you installed.
3. Formatting and linting: tools such as Black or Ruff keep code style consistent and flag common mistakes on save.
4. Type checking: mypy or Pyright check your type hints (covered in chapter 3).

Summary

• Start new work on Python 3 and confirm the install with python3 --version.
• Use the REPL for quick experiments and .py files for real programs, guarded by if __name__ == "__main__":.
• Give every project its own environment with python3 -m venv .venv and activate it before working.
• Install packages with python -m pip install, and make sure your editor uses the environment's interpreter.