Setup & Your First Program

What is Python?

Python is a general-purpose, high-level, interpreted programming language created by Guido van Rossum and first released in 1991. It is consistently ranked as one of the world's most popular languages — used in web development, data science, machine learning, automation, scientific computing, and far more.

"Interpreted" means Python reads and runs your source code line by line at runtime, rather than compiling it to machine code ahead of time. This makes development fast: change a line, run again, see the result immediately.

The Zen of Python

Python has an official set of guiding principles. Run import this in the REPL to see all 19 aphorisms. Here are the most important ones to remember as a beginner:

Installing Python

Download Python 3.11 or later (3.12 recommended as of 2024). Python 2 reached end-of-life in 2020 — always use Python 3.

python --version
# Python 3.12.0

brew install python3

python3 --version
# Python 3.12.0

sudo apt update
sudo apt install python3 python3-pip

sudo dnf install python3

python3 --version
# Python 3.12.0

The Python REPL

The REPL (Read-Eval-Print Loop) is an interactive Python shell built into the interpreter. You type a statement, Python reads it, evaluates it, prints the result, and loops back for the next input.

Start it from your terminal:

python3

You'll see the >>> prompt. Try these:

Your First Script

While the REPL is great for experiments, real programs live in .py files. Follow these steps to create and run your first script:

1. Open any text editor (VS Code, Notepad, nano — anything).
2. Create a new file called hello.py.
3. Type the code below and save the file.
4. Open your terminal, navigate to the folder containing hello.py, and run it.

# My first Python program
print("Hello, World!")
print("Python is fun.")
print(1 + 1)

python3 hello.py
# Hello, World!
# Python is fun.
# 2

Python executes the file from top to bottom, one statement at a time. Every line you see above runs in order — line 1 first, then line 2, then line 3.

Understanding print()

print() is a built-in function — one of many tools Python provides out of the box. It writes its argument(s) to standard output (your terminal).

print("Hello")               # prints a string
print(42)                    # prints a number
print(3.14)                  # prints a float
print("Sum:", 10 + 5)        # prints label + result → Sum: 15
print("a", "b", "c")        # multiple values → a b c
print("line1\nline2")        # \n is a newline character

Comments

A comment is text in your code that Python completely ignores. Anything after a # character on a line is a comment.

# This entire line is a comment — Python skips it

print("visible")  # This part is a comment; print() still runs

# Comments explain WHY your code does something,
# not just what it does. Good comments save future-you.

# TODO: add error handling here
# FIXME: this formula is wrong for negative inputs

Code Style: PEP 8

PEP 8 is Python's official style guide. You don't need to memorise it now, but three rules matter from day one:

• Use 4 spaces per indentation level (not tabs).
• Keep lines to 79 characters or fewer.
• Put a space around operators: x = 1 + 2, not x=1+2.

Most editors (VS Code, PyCharm) will auto-format Python for you. The tool black can reformat an entire project automatically — we'll use it later.

print("Alice")
print(6 * 7)            # 42
print("I am learning Python.")

>>> 17 % 3       # modulo (remainder)
2
>>> "ha" * 3
'hahaha'
>>> type(42)
<class 'int'>
>>> type("hello")
<class 'str'>
>>> type(3.14)
<class 'float'>
>>> 2 ** 10      # 2 to the power 10
1024

# "Readability counts."
#
# I chose this one because the whole point of code is
# communication — first with the computer, then with
# other humans (including future-me). If it's hard to
# read, it's hard to debug and hard to improve.

import this   # prints the full Zen of Python