Is the Functions lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Functions runs real Python in your browser via Pyodide.

How long does the Functions lesson take?

About 8 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Functions lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Functions lesson?

Beginner level. No prior Python required — good starting point.

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Beginner·8 min·basics · functions

Functions

You’ll be able to

Define functions with def and return values with return
Pass positional and keyword arguments
Set default parameter values
Understand variable scope: local vs global

Why this matters

Functions are the unit of reuse and testing in every Python codebase. Frameworks like Flask, FastAPI, and pytest are built on decorated functions, and knowing default-argument traps is a common senior-level interview question.

Common pitfalls

Using mutable defaults: def f(x=[]). The list persists across calls; use x=None then x = x or [] inside.
Forgetting return. A function without it returns None, silently breaking any caller that expected data.
Mixing positional and keyword args wrong: f(a=1, 2) raises SyntaxError. Positional must come before keyword.

A function is a named, reusable block. def defines one. return hands a value back.

Default arguments

excited=False is a default. Callers can override it by name.

Try it

Write a function area(width, height) that returns the area of a rectangle.
Add a default for height so calling area(5) returns the area of a 5×5 square.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define a function area(width, height) that returns (not prints) the area of a rectangle. Then print area(4, 5). Expected last line: 20.
Exercise 2
Rewrite area with a default — if height is omitted, use the same value as width (a square). Then print area(6) (should be 36) and area(4, 5) (should be 20).
Exercise 3
Write a function split_name(full) that takes a string like "Ada Lovelace" and returns a tuple (first, last). Then print split_name("Grace Hopper"). Expected includes: ('Grace', 'Hopper').

← Previousenumerate, zip, range Next →Nested Data

Functions

You’ll be able to

Define functions with def and return values with return
Pass positional and keyword arguments
Set default parameter values
Understand variable scope: local vs global

Why this matters

Common pitfalls

Using mutable defaults: def f(x=[]). The list persists across calls; use x=None then x = x or [] inside.
Forgetting return. A function without it returns None, silently breaking any caller that expected data.
Mixing positional and keyword args wrong: f(a=1, 2) raises SyntaxError. Positional must come before keyword.

A function is a named, reusable block. def defines one. return hands a value back.

Default arguments

excited=False is a default. Callers can override it by name.

Try it

Write a function area(width, height) that returns the area of a rectangle.
Add a default for height so calling area(5) returns the area of a 5×5 square.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Define a function area(width, height) that returns (not prints) the area of a rectangle. Then print area(4, 5). Expected last line: 20.
Exercise 2
Rewrite area with a default — if height is omitted, use the same value as width (a square). Then print area(6) (should be 36) and area(4, 5) (should be 20).
Exercise 3
Write a function split_name(full) that takes a string like "Ada Lovelace" and returns a tuple (first, last). Then print split_name("Grace Hopper"). Expected includes: ('Grace', 'Hopper').