Lesson 1: Hello, Python! Setup & Your First Program

What is Python?

Python is a general-purpose, high-level programming language created by Guido van Rossum and first released in 1991. It was designed with one guiding philosophy: code should be readable and easy to write. Because Python reads almost like plain English, you can focus on thinking about the problem instead of fighting the syntax. Today Python is the #1 language for beginners, data science, machine learning, artificial intelligence, and web back-ends — which is exactly why this course starts here.

Why Is Python Everywhere in 2026?

• Data Science & AI: Pandas, NumPy, PyTorch, and TensorFlow are all Python-first. If a company trains a model, it is almost certainly written in Python.
• Web Development: Django, FastAPI, and Flask power millions of back-ends.
• Automation & Scripting: Python automates boring tasks — renaming files, scraping websites, cleaning data, and gluing other tools together.
• Education: Its gentle learning curve makes it the default first language in universities worldwide.
• Huge Ecosystem: The Python Package Index (PyPI) hosts over half a million free packages.

Mental model: think of Python as a power tool with a friendly interface — the underlying machinery (C code, memory management) is hidden, so you operate at the level of ideas, not hardware.

What You'll Learn in This Lesson

• Install Python and verify it works on your machine
• Write and run your very first program
• Understand the difference between the REPL and script files
• Master the print() function and its formatting options
• Write comments to document your code
• Understand why indentation is not optional in Python

Installing Python (Step by Step)

1. Go to python.org/downloads and download the latest stable version (3.11 or newer). Version 3.12+ is recommended — it is faster and has the newest features.
2. Run the installer. Important: tick the checkbox "Add Python to PATH" before clicking Install Now — this lets you run python from any terminal. On macOS/Linux you can also install via brew install python or your package manager.
3. Open a terminal (Command Prompt on Windows, Terminal on macOS/Linux) and verify the installation:

python --version
# Output: Python 3.12.4

If you see a version number, Python is ready. If you see "python is not recognized" or "command not found", you missed the PATH checkbox (Windows) or Python isn't on your PATH (macOS/Linux) — reinstall and tick it, or use python3 on macOS/Linux.

Windows tip: on modern Windows you can also type py instead of python — the Python Launcher finds the right version for you.

The REPL vs Script Files

There are two ways to run Python code:

The REPL reads your input, evaluates it, prints the result, and loops back for more — an instant playground. Script files are how real software is built: you save your code, run the whole file, and share it with others.

Interactive Python in the browser: if you don't want to install anything yet, python.org/shell and replit.com give you a working Python environment in your browser in seconds.

Your First Program

The most famous first program in the world is exactly one line:

print("Hello, world!")

Save it as hello.py and run python hello.py. The output is:

Hello, world!

The print() Function — Your First Tool

print() writes text to the console. Whatever you put inside the parentheses is shown to the user. It has a few powerful options:

You can print any number of values in one call, mixing strings and numbers freely:

print("I am", 25, "years old")
print("A", "B", "C", sep=" - ")
print("Line one", end=" | ")
print("still line one")

I am 25 years old
A - B - C
Line one | still line one

Why it matters: sep and end might look trivial, but they are the difference between pretty, readable output and a wall of mangled text. Professional scripts use them constantly.

Comments — Notes for Humans

A comment is a note to yourself or other programmers. Python ignores everything after a # on that line:

# This is a comment — Python skips it entirely
print("This runs")  # Comments can also go after code
# print("This line is commented out, so it will NOT run")

Comments are how you explain why code exists, not what it does — the code itself shows the what. Good comment habits:

• Comment the why ("we multiply by 1.18 to add 18% GST"), not the obvious what ("this adds two numbers").
• Use comments to temporarily disable code while debugging.
• Keep comments short and current — stale comments mislead worse than none.
• Python also supports multi-line docstrings ("""...""") for documenting functions and classes — you will use them heavily from Lesson 12 onward.

Indentation is Everything

Most languages group blocks of code with curly braces ({ }). Python uses indentation (spaces or tabs) instead. This is a feature, not a quirk: it forces every programmer to write neatly formatted code.

• A block of code is indented 4 spaces by default (this is the PEP 8 standard).
• Never mix tabs and spaces — it causes an IndentationError.
• Inconsistent indentation is the #1 cause of "my code suddenly broke" for beginners.

if True:
    print("Inside the if-block")   # 4 spaces
    print("Still inside")           # same indent = same block
print("Back at top level")          # no indent = outside

How to Visualize Code Execution

Imagine Python reading your file top to bottom, one line at a time, like a person following a recipe. Every print() is a "shout" to the console — the order of the shouts is exactly the order of execution. There is no jumping ahead and no skipping lines (until we learn about loops and functions in later lessons).

A beginner debugging ritual: before running code, predict the output by reading line by line out loud. Then run it. If your prediction was wrong, you just found a misunderstanding — and that is a great thing, because now you know exactly what to study.

Understanding Your First Errors

Errors are not failures — they are the interpreter telling you exactly what it needs. The two you will meet first:

Read the last line of a traceback first — it names the error type and the line number. The huge stack of text above it is context; the punchline is at the bottom.

Common Mistakes to Avoid

• Mistake: print "Hello" (forgetting parentheses) — Fix: print("Hello"). Python 3 requires parentheses; the old Python 2 syntax no longer works.
• Mistake: Mixing tabs and spaces for indentation — Fix: configure your editor to convert tabs to 4 spaces (VS Code: "Editor: Insert Spaces").
• Mistake: Naming a file print.py or math.py — Fix: avoid names that collide with Python keywords or built-in modules.
• Mistake: Skipping the "Add Python to PATH" checkbox — Fix: reinstall and tick it, or use py (Windows) / python3 (macOS/Linux).
• Mistake: Typing python inside the REPL to run a script — Fix: exit the REPL (exit() or Ctrl+Z) first, then run python hello.py.

Professional Tips & Tricks

• Use 4 spaces for indentation — never mix tabs and spaces.
• Name your files with lowercase letters and underscores: hello_world.py.
• Use print() freely while learning — it is your flashlight for seeing what code does.
• Install a good editor: VS Code (free) with the Python extension gives you syntax highlighting, autocomplete, and one-click run buttons.
• Keep a terminal window open beside your editor — running code every few lines is how you learn fastest.

Key Takeaways

• Python is readable, popular, and the best first language to learn.
• The REPL is for experiments; .py script files are for real programs.
• print() displays output and supports sep, end, file, and flush.
• Comments (#) document code and are ignored by Python.
• Consistent 4-space indentation defines code blocks — and is non-negotiable.
• Errors like SyntaxError and NameError tell you exactly what to fix — read the last line first.

Next up: Variables, data types, and how Python reads input from the user.