Functions in Python

Imagine you're preparing a complex dish. Instead of coming up with a recipe from scratch each time, you can use an existing one. Functions in Python work in a similar way! 🧩

Functions are like ready-made recipes for your code. They allow you to create blocks of code that can be used multiple times, making your programs more organized and easier to understand.

What are functions?

A function is a named block of code that performs a specific task and can be called from other parts of the program.

Functions help to:

Organize code into logical blocks
Avoid code repetition
Make the program more readable
Simplify debugging and maintenance

Creating and calling functions

Basic syntax

A function in Python is created using the keyword def:

Python 3.13
# Simple function example
>>> def greet():
...     print("Hello, world!")

# Function call
>>> greet()
Hello, world!

In this example:

def indicates that we're creating a function
greet is the function name
() are brackets for parameters (empty for now)
: is the colon after the definition
Function body with indentation

Functions with parameters

Functions can accept parameters — values that are passed to the function when it's called:

Python 3.13
# Function with parameters
>>> def greet(name):
...     print(f"Hello, {name}!")

# Function call with an argument
>>> greet("Anna")
Hello, Anna!
>>> greet("Peter")
Hello, Peter!

Return values

Functions can return the result of their work using the return operator:

Python 3.13
# Function with a return value
>>> def add(a, b):
...     return a + b

# Using the function result
>>> result = add(5, 3)
>>> print(f"5 + 3 = {result}")
5 + 3 = 8

Function parameters

Positional and named parameters

Python supports two ways of passing arguments to a function:

Python 3.13
# Function with multiple parameters
>>> def describe_pet(animal_type, pet_name):
...     print(f"I have a {animal_type} named {pet_name}.")

# Positional arguments
>>> describe_pet("dog", "Rex")
I have a dog named Rex.

# Named arguments
>>> describe_pet(pet_name="Fluffy", animal_type="cat")
I have a cat named Fluffy.

Default parameters

You can set default values for parameters:

Python 3.13
# Function with default parameters
>>> def describe_pet(pet_name, animal_type="dog"):
...     print(f"I have a {animal_type} named {pet_name}.")

# Using the default value
>>> describe_pet("Rex")
I have a dog named Rex.

# Overriding the default value
>>> describe_pet("Fluffy", "cat")
I have a cat named Fluffy.

Arbitrary number of arguments

Sometimes you don't know in advance how many arguments will be passed to the function. For this, special parameters are used:

Python 3.13
# Function with an arbitrary number of positional arguments
>>> def make_pizza(*toppings):
...     print("Making a pizza with the following toppings:")
...     for topping in toppings:
...         print(f"- {topping}")

>>> make_pizza("pepperoni")
Making a pizza with the following toppings:
- pepperoni
>>> make_pizza("mushrooms", "green peppers", "extra cheese")
Making a pizza with the following toppings:
- mushrooms
- green peppers
- extra cheese

# Function with an arbitrary number of named arguments
>>> def build_profile(first, last, **user_info):
...     profile = {}
...     profile['first_name'] = first
...     profile['last_name'] = last
...     for key, value in user_info.items():
...         profile[key] = value
...     return profile

>>> user_profile = build_profile('Anna', 'Smith',
...                            location='New York',
...                            field='programming')
... print(user_profile)
{'first_name': 'Anna', 'last_name': 'Smith', 'location': 'New York', 'field': 'programming'}

Understanding check

Let's check how well you've understood the topic of functions:

What will the following code output?

Python 3.13
def mystery(x, y=2):
    return x * y

result = mystery(3)
print(result)