Chapter 3.1: Defining Functions

In Python, functions are a way to group code fragments that perform a specific task. Functions promote code reusability, make the code easier to read, and allow for more manageable code.

Syntax of Function

Here is the basic syntax for defining a function in Python:

def function_name(parameters):
    """docstring"""
    statement(s)

• The keyword def starts the function definition, followed by the function name.
• Parentheses contain the function’s parameters, also called arguments. Multiple parameters are separated by commas.
• A colon (:) follows the parentheses, and the indented lines after the colon are the function’s body.
• The optional docstring (short for documentation string) describes what the function does. Although optional, documentation is a good programming practice.
• The function body contains one or more valid Python statements. These statements must be indented.

Here’s an example of a function that adds two numbers:

def add_numbers(num1, num2):
    """This function adds two numbers."""
    return num1 + num2

To use this function, you would call it, passing two numbers as arguments. For example, add_numbers(2, 3) would return 5.

The return Statement

The return statement is used to exit a function and go back to the place where it was called. It can also send a result back to the caller. For example, in our add_numbers function above, return num1 + num2 sends the sum of the two numbers back to the caller.

A function without a return statement returns None.

Arguments and Parameters

You can call a function using the following types of arguments:

• Required arguments: the function arguments that must be passed in the correct positional order.
• Keyword arguments: allows you to skip arguments or place them out of order because Python interpreter is able to use the keywords provided to match the values with parameters.
• Default arguments: a default value is assigned to the argument which is used if the function is called without passing that particular argument.
• Variable-length arguments: processes a function for more arguments than you specify while defining the function.

By understanding the basic structure of a function and how to define one, you’ll be well on your way to writing cleaner, more reusable code in Python. Functions are a foundational component of any programming language and mastering them is crucial to becoming an efficient Python programmer.