What Is a Function in Python?

A function in Python is a reusable block of code that does a specific task. You can run it when needed, and it can optionally send a value back.

Functions are one of the main ways to organize code. They help you group related steps together and avoid writing the same code again and again.

Simple definition

A function is:

A named block of code
A way to group steps that belong together
Something you can run more than once
A tool for avoiding repeated code

Here is a very simple example:

def say_hello():
    print("Hello")

say_hello()

Output:

Hello

This example shows the two main parts:

Defining a function with def
Calling the function by writing its name with parentheses

The function does not run when Python first sees def say_hello():. It runs only when you call say_hello().

Why functions are useful

Functions are useful because they help you:

Make code easier to read
Reduce repetition
Break large problems into smaller parts
Reuse the same logic in different places

For example, without a function, you might repeat the same lines several times. With a function, you write the code once and call it whenever you need it.

How a function works

A basic function works like this:

You define a function with def
You give the function a name
You put the code for that task inside the function
The indented code runs when the function is called

Example:

def show_message():
    print("This is inside the function")

show_message()

Output:

This is inside the function

Key parts:

def starts the function definition
show_message is the function name
() are used in both the definition and the call
The indented line is the function body

If you want a fuller introduction, see Python functions explained.

Functions can take input

Functions can work with different data by taking input.

Inputs in the function definition are called parameters
Values you pass when calling the function are called arguments
This lets one function work with different values

Example:

def greet(name):
    print("Hello,", name)

greet("Maya")
greet("Leo")

Output:

Hello, Maya
Hello, Leo

Here:

name is a parameter
"Maya" and "Leo" are arguments

This is useful because you do not need a separate function for every person or value.

Functions can return output

A function can also send a value back using return.

return gives a value back to the program
The returned value can be stored in a variable
Not every function needs return
print() shows something on the screen, but return gives a value back to your code

Example:

def add_numbers(a, b):
    return a + b

result = add_numbers(2, 3)
print(result)

Output:

In this example:

The function calculates a + b
return a + b sends the result back
That result is stored in result

This is different from:

def add_and_print(a, b):
    print(a + b)

value = add_and_print(2, 3)
print(value)

Output:

5
None

Why does None appear? Because the function printed the result, but it did not return one.

If you want to learn this idea in more detail, see what is a return value in Python.

Built-in functions and your own functions

Python already includes many built-in functions, such as:

print()
len()
input()

Example:

text = "Python"
print(len(text))

Output:

You can also create your own functions with def.

Both built-in functions and your own functions are usually called with parentheses. For example:

print("Hi")
len("cat")
say_hello()

If you want to understand one common built-in function better, see Python print() function explained.

What this page does not cover in depth

This page explains the basic idea of a function. It does not go deeply into:

Detailed parameter rules
Default arguments and keyword arguments
Lambda functions
Advanced function concepts

If your next goal is to make one yourself, see how to create a simple function in Python.

Common mistakes

Beginners often run into these problems when learning functions:

Thinking a function runs as soon as it is defined
Forgetting to call the function after defining it
Confusing print() with return
Forgetting the parentheses when calling a function
Using wrong indentation inside the function body

Example of a function that is defined but not called:

def say_hello():
    print("Hello")

This code creates the function, but it does not produce output yet. To run it, you must call it:

def say_hello():
    print("Hello")

say_hello()

You can also use a few quick checks while learning:

print(my_function)
print(my_function())
help(print)
type(print)
type(my_function)

What these show:

print(my_function) shows that the function exists
print(my_function()) calls the function and prints its return value
help(print) shows help for the built-in print() function
type(print) shows that print is a built-in function
type(my_function) shows that your function is a function object

Be careful with print(my_function()): it runs the function. If the function only prints something and does not return a value, you may also see None.

What Is a Function in Python?

Simple definition

Why functions are useful

How a function works

Functions can take input

Functions can return output

Built-in functions and your own functions

What this page does not cover in depth

Common mistakes

FAQ

What is a function in simple words?

What is the difference between defining and calling a function?

Does every function need a return statement?

Is `print()` a function?

Why should beginners use functions?

See also