Getting Started with Python

Introduction

Python is an open-source, high-level, dynamically typed programming language created by Guido van Rossum and first released in 1991. It is one of the most popular languages in the world, dominating web development, data science, machine learning, scripting, and automation. Python's philosophy emphasizes readability, explicit beats implicit, and there should be one — and preferably only one — obvious way to do it.

Installing Python

Python ships pre-installed on most macOS and Linux systems, but those system Python versions are often outdated and should not be used for development. Install a current Python (3.12+ recommended).

Step 1: Download Python

Step 1: Visit python.org

Navigate to https://www.python.org/downloads/ and download the latest stable Python 3 release for your platform.

Step 2: Choose Your Platform

Platform-Specific Installation

macOS

# Recommended: Homebrew
$ brew install python@3.12

# Or use pyenv for multiple Python versions
$ brew install pyenv
$ pyenv install 3.12.3
$ pyenv global 3.12.3

Windows

Run the .exe installer from python.org and check “Add python.exe to PATH” on the first screen. Then open a new Command Prompt or PowerShell.

> py --version
Python 3.12.3

> py -m pip --version
pip 24.0 from C:\Python312\Lib\site-packages\pip (python 3.12)

Linux

# Debian / Ubuntu
$ sudo apt update
$ sudo apt install -y python3 python3-venv python3-pip

# Fedora
$ sudo dnf install -y python3 python3-pip

# Arch
$ sudo pacman -S python python-pip

Verify Installation

$ python3 --version
Python 3.12.3

$ python3 -m pip --version
pip 24.0

$ python3 -c "import sys; print(sys.executable)"
/usr/local/bin/python3.12

Virtual Environments

A virtual environment is an isolated Python installation per project. It prevents dependency conflicts between projects and is the foundation of every professional Python workflow. Never install packages globally with sudo pip install.

Why venv Matters

Create and Activate a venv

Step 1: Create the environment

$ python3 -m venv .venv

This creates a .venv/ directory in the current folder containing a fresh Python interpreter.

Step 2: Activate it

# macOS / Linux
$ source .venv/bin/activate
(.venv) $

# Windows (PowerShell)
PS> .\.venv\Scripts\Activate.ps1
(.venv) PS>

The shell prompt now shows (.venv). Inside the venv, python and pip point to the isolated interpreter.

Step 3: Install packages

(.venv) $ pip install requests rich
Successfully installed certifi-2024.7.4 charset-normalizer-3.3.2 \
    idna-3.7 markdown-it-py-3.0.0 pygments-2.18.0 requests-2.32.3 rich-13.7.1

(.venv) $ pip list
Package           Version
----------------- --------
certifi           2024.7.4
requests          2.32.3
rich              13.7.1

Step 4: Freeze and deactivate

(.venv) $ pip freeze > requirements.txt
(.venv) $ deactivate
$

Anyone can reproduce your environment with pip install -r requirements.txt inside their own venv.

Modern Alternatives

Your First Python Program

Hello, World!

Step 1: Create a project

$ mkdir hello-python
$ cd hello-python
$ python3 -m venv .venv
$ source .venv/bin/activate

Step 2: Create main.py

def main():
    print("Hello, Python!")

if __name__ == "__main__":
    main()

Understanding the Code

1. Function Definition

def main():
    ...

The def keyword defines a function. Indentation — not braces — marks the function body. Four spaces is the convention (PEP 8). Mixing tabs and spaces is a syntax error in Python 3.

2. The print() Built-in

print("Hello, Python!")

print() writes to standard output and adds a newline by default. It accepts any number of arguments, separates them with spaces, and supports sep=, end=, and file= keyword arguments.

3. The if __name__ == "__main__" Guard

if __name__ == "__main__":
    main()

When you run python main.py, Python sets __name__ to "__main__". When the same file is imported as a module, __name__ is the module name. This idiom lets a file be both a runnable script and an importable module.

Running Your Program

(.venv) $ python main.py
Hello, Python!

The Interactive REPL

Run python with no arguments to get an interactive prompt — a read-eval-print loop where you can experiment line by line.

(.venv) $ python
Python 3.12.3 (main, ...) on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 2 + 2
4
>>> name = "Robbie"
>>> f"Hello, {name}!"
'Hello, Robbie!'
>>> exit()

Essential Python Commands

Setting Up Your Editor

VS Code

Step 1: Install VS Code

Download from https://code.visualstudio.com/.

Step 2: Install the Python extension

Install Python by Microsoft — it bundles Pylance (type checker / IntelliSense) and a debugger.

Step 3: Recommended settings

{
    "python.defaultInterpreterPath": ".venv/bin/python",
    "python.analysis.typeCheckingMode": "basic",
    "editor.formatOnSave": true,
    "[python]": {
        "editor.defaultFormatter": "charliermarsh.ruff",
        "editor.codeActionsOnSave": {
            "source.organizeImports": "explicit",
            "source.fixAll": "explicit"
        }
    }
}

PyCharm (JetBrains)

PyCharm Community is free and provides excellent refactoring, debugging, and database tools out of the box. Open a folder, point it at .venv/bin/python, and you're set.

Essential Python Tools

(.venv) $ pip install ruff mypy pytest ipython

# Lint and auto-fix (replaces flake8, isort, black)
(.venv) $ ruff check .
(.venv) $ ruff format .

# Static type checking
(.venv) $ mypy main.py

# Run tests
(.venv) $ pytest

Project Structure

Simple Script

hello-python/
├── .venv/                # virtual environment (gitignored)
├── .gitignore
├── README.md
├── requirements.txt      # dependencies
└── main.py               # entry point

Standard Application Layout

myapp/
├── .venv/
├── .gitignore
├── README.md
├── pyproject.toml        # project metadata + dependencies
├── src/
│   └── myapp/
│       ├── __init__.py
│       ├── __main__.py   # python -m myapp
│       ├── cli.py
│       └── core.py
└── tests/
    ├── __init__.py
    └── test_core.py

Expanded Example: Interactive Greeter

A more substantial program demonstrating input, conditionals, f-strings, and the standard library:

from datetime import datetime
import time


def greeting_for(hour: int) -> str:
    # Pick a greeting based on the hour of day.
    if hour < 12:
        return "Good morning"
    if hour < 18:
        return "Good afternoon"
    return "Good evening"


def main() -> None:
    print("Welcome to the Python Greeter!")
    print("=" * 32)

    name = input("What's your name? ").strip().title()
    greeting = greeting_for(datetime.now().hour)
    print(f"\n{greeting}, {name}! 👋")

    facts = [
        "🐍 Python was named after Monty Python's Flying Circus.",
        "📊 Pandas, NumPy, and PyTorch power most data and ML work.",
        "⚡ asyncio lets one thread juggle thousands of connections.",
        "🧪 pytest is the de-facto testing framework.",
    ]
    print("\nHere are some Python facts:")
    for i, fact in enumerate(facts, start=1):
        print(f"{i}. {fact}")
        time.sleep(0.4)

    print("\nHappy coding with Python! 🎉")


if __name__ == "__main__":
    main()

Running the Program

(.venv) $ python greeter.py
Welcome to the Python Greeter!
================================
What's your name? robbie

Good morning, Robbie! 👋

Here are some Python facts:
1. 🐍 Python was named after Monty Python's Flying Circus.
2. 📊 Pandas, NumPy, and PyTorch power most data and ML work.
3. ⚡ asyncio lets one thread juggle thousands of connections.
4. 🧪 pytest is the de-facto testing framework.

Happy coding with Python! 🎉

Common Pitfalls and Solutions