8. Functions

Reusing code is essential in programming. That’s why functions exist. Functions are a convenient way to divide your code into useful blocks, allowing us to order our code, make it more readable, reuse it and save some time. Also functions are a key way to define interfaces so programmers can share their code.

8.1. Defining a Function

Defining a function in Python is as easy as:

def sample_function():
    print("Hello from a function!")

The function sample_function doesn’t accept any parameters. In fact, you can only call the function, which prints Hello from a function! Calling the function is as easy as:


8.2. Positional Arguments

If you want your function to accept arguments, you can add them to the functions definition.

def greet(name):
    print(f"Hello {name}!")

Calling the function with your name results in a simple greet to yourself:

>>> greet("Florian")
Hello Florian!

Note: Did you notice the little f in front of the string in the print function? This turns the string into an f-string. They exist since Python 3.6 and allow you to format strings in a pretty straightforward way. Before that, one made use of the built-in format method belonging to the str type.

This kind of arguments is called positional arguments as Python maps the values you provide to the accepting arguments by position.

8.3. Keyword Arguments

Besides positional arguments Python has another type of arguments: Keyword arguments. As the name suggests this means, that you specify a value for the argument using a keyword. This allows you to specify default values if no value is provided for a certain argument as well as to provide values for the arguments independent from the position.

>>> def greet(name="Sam", location="Hamburg"):
...     print(f"Hey {name}! You are from {location}, right?")
>>> greet()
Hey Sam! You are from Hamburg, right?
>>> greet(name="Florian")
Hey Sam! You are from Hamburg, right?
>>> greet(location="Berlin", name="Florian")
Hey Florian! You are from Berlin, right?

8.4. Return Values

You can return any value/object from a function. Simply use the return keyword:

>>> def square(number):
...     return number ** 2
>>> square(4)

8.5. Further Resources