Summary  
In Python, functions without an explicit return statement default to returning None, a pattern typically used when a function performs side effects rather than producing a value.  

General domain of usage  
Data persistence

In Python, `None` is a special object that represents the absence of a value. **If we don't specify any return value, `None` is used by default**. It is commonly used to indicate that a function does not return any meaningful result. Let's consider different cases when a `None` return value can be used:

## First Case
When the function does not have a specific result or value to return:

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

result = greet('Alice')
print(result)

In this case, the `greet()` function simply prints a greeting message without returning a specific value. The default return value is `None`.

## Second Case
When a function is used for its side effects or actions rather than a return value:

def save_data(data):
    # Save the data to a file or database
    # ...
	print('Data was saved succesfully!')

data = []
result = save_data(data)
print(result)  # output: `None`

In this example, the function `save_data()` performs actions to save data but does not return a specific result. The return value is `None`, but the focus is on the side effect of saving the data.

You'll have a solid grasp of how important functions are in Python, no matter your field in computer science. We'll cover creating functions, defining arguments, handling return values, and exploring their use in Python.

We will consider what a function in Python and some basic techniques used to create functions is: what are the arguments of the function, what is the return value of the function, what is the body of the function, and how to use the function in the code.

Learn how to define functions in Python using positional and optional arguments. Master the flexibility of creating functions that adapt to different input scenarios.

Learn to work with arbitrary arguments and keyword arguments in Python to handle variable numbers of inputs. Understand how arbitrary arguments manage flexible positional inputs and how keyword arguments organize named arguments into a dictionary for efficient function design.

The return value of a function is the direct result of the execution of the function, which we can use in the program. There are several types of function return values - we will consider the most commonly used.

Explore the power of recursion and lambda functions in Python. Understand how recursion allows a function to call itself for solving complex problems, and how lambda functions provide a concise way to define anonymous functions for simple tasks.

None Return Value

First Case

Second Case