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Python Introduction

Python is a simple, readable programming language that's great for beginners and powerful enough for production systems. It's used everywhere — websites, AI, data science, scripting, automation.

Why Python?

  • Easy to read — looks almost like English.
  • No semicolons or curly braces — uses indentation (spaces) instead.
  • Huge library ecosystem — there's a library for almost anything.
  • Free and open-source.

Your first Python program

Click ▶ Run to see it execute in your browser:

print("Hello, World!")

Expected output: 

Hello, World!

print(...) is a built-in function. Anything inside the brackets gets shown on screen.

print("Hello", "Alice", 25, "years old")

print separates each argument with a space by default.

Change the separator with sep=:

print("Hello", "Alice", "Bob", sep=" -- ")

Change what's at the end of the line with end=:

print("Loading", end="...")
print("done")

Indentation matters

Unlike most languages, Python uses indentation (the spaces at the start of a line) to group code. No curly braces.

if 5 > 3:
    print("Five is bigger")
    print("This is also inside the if")
print("This is outside the if")

Rule: indentation must be consistent — most people use 4 spaces. Mixing tabs and spaces causes errors.

Wrong indentation gives an IndentationError:

if 5 > 3:
print("Wrong indentation")

Run that — it will fail with IndentationError. Now fix it (add 4 spaces in front of print) and re-run.

Practice

What does this print?

Expected: Hi-there-friend

print("Hi", "there", "friend", sep="-")

Make the loading message print on one line

Expected: Loading...done

print("Loading")     # bug: should suppress the newline with end="..."
print("done")

Quiz — Quick check

What you remember

Q1. What separates Python blocks of code (e.g. the body of an if)?

  • Curly braces { }
  • Semicolons ;
  • Indentation (spaces or tabs)
  • Parentheses ( )

Why: Python uses indentation to define blocks. Most other languages use {} — Python's choice makes code visually consistent.

Q2. What does print("a", "b", "c", sep="|") produce?

  • a b c
  • a|b|c
  • abc
  • a, b, c

Why: sep controls what's placed between the arguments. The default is a single space.

Q3. Why does the buggy snippet if 5 > 3:\nprint("hi") fail?

  • print is misspelled
  • Missing ; after if 5 > 3:
  • The body of if must be indented
  • 5 > 3 returns None

Why: Python requires the body of any block (if, for, def, …) to be indented. Without indentation, Python doesn't know what's inside the if.

Common doubts

Do I need to install anything to start writing Python?

For real projects, yes — download Python from python.org. For these chapters, you don't need to install anything; the ▶ Run button uses Pyodide (Python compiled to WebAssembly) right in your browser.

Does Python need semicolons at the end of each line?

No. Python uses newlines to end a statement. You can put multiple statements on one line with ; (e.g. a = 1; b = 2) but it's considered bad style — write one statement per line.

Why 4 spaces specifically for indentation?

The Python style guide (PEP 8) recommends 4 spaces. The language only cares that indentation is consistent within a block — any consistent number works, but 4 spaces is what almost every Python codebase uses. Don't mix tabs and spaces in the same file; mixing causes IndentationError.

What's next

→ Variables, Types & Literals