Functions

Lesson

Part 1 — what a function is, the three types of functions, and how to define, call and return from one.

In this lesson you will

Distinguish built-in functions, functions from modules and user-defined functions
Use functions of the math, random and statistics modules after importing them
Define and call a function, and explain the difference between return and print

Part 1 at a glance

Concept

Why use functions?

A function is a named block of statements that performs one task and runs only when it is called.

Table of the benefits of functions: reuse, modularity, readability, easier testing and less repetition
Benefit	What it means
Reuse	Write the code once, call it many times
Modularity	Split a big program into small, named parts
Readability	A good name such as `gst()` explains the code
Easier testing	Check each function on its own
Less repetition	A change is made in one place only

Concept

Table of the three types of functions: built-in functions such as len and print, functions from modules such as math sqrt, random randint and statistics mean, and user-defined functions written with def
Type	Where it comes from	Examples
Built-in	Always available	`len()`, `print()`, `max()`, `round()`, `type()`
From a module	Needs `import` first	`math.sqrt()`, `random.randint()`, `statistics.mean()`
User-defined	Written by you with `def`	`def area(l, w): ...`