In Python, a function is a named block of reusable code that performs a specific task. It is a fundamental concept in programming and plays a crucial role in organizing and structuring code. There are several reasons to use functions in Python:

1. Code Reusability: Functions allow us to define reusable blocks of code that can be called multiple times from different parts of the program. This promotes code reusability and avoids duplicating code.
2. Modularity: Functions help break down complex tasks into smaller, more manageable code units. Each function can focus on a specific task, making the overall code more modular, easier to understand, and maintainable.
3. Organization and Readability: Functions help organize code by grouping related operations together. By dividing a program into functions, it becomes easier to understand the flow of the program and improves the readability of the code.
4. Code Maintainability: Functions promote code maintainability by allowing developers to make changes or updates in one place (the function) rather than searching for and modifying the same code in multiple places. It reduces the chances of introducing errors and makes it easier to update the behavior of the program.

Defining a function

To define the most simple function in Python, we have to:

1. Firstly, def keyword has to be used.
2. Secondly, we have to specify the function name - a unique identifier that represents the function. It follows variables' naming conventions, such as using lowercase letters and underscores ( this convention is called snake_case).
3. We have to use (): after the function name - in these brackets, we specify the function's arguments (parameters).
4. Then, we specify the function body with the indented new line.
5. After the function body, we can specify the return value of the function using different keywords such as return, assign, or yield.
6. Finally, we can use functions in code. To do in, we have to call function using its name and specifying its parameters.

Let's look at the example. We will create a simple function to print 'Hello, world!' in the console and use this function in code by calling it.

Note

You may have noticed that we used print() function as the body of print_hello(). It is a built-in function that prints the text in the console. It has a string as and arguments - we specify this string in the () brackets.

You may ask why this function has no return value if it prints the message in the console.
In Python, a function can perform a task (in this case, printing a message) without necessarily returning a value. The presence or absence of a return statement in a function depends on whether the function is intended to produce an output (return a value) that can be used in other parts of the program.
The print_hello() function prints a string in the console, but we can't use this string in the program, manipulate it, or use it as an input for other functions. So, the print_hello() function has no return value.