Functions: Definition and Usage in Python

Functions are a fundamental building block in Python that allows you to encapsulate code for reuse. Functions help organize code into reusable, logical units, improving readability and maintainability.

Subtopic 1: Defining and Calling Functions

A function in Python is defined using the def keyword, followed by the function name, parentheses, and an optional block of code that the function will execute.

Syntax for Defining a Function

def function_name(parameters):
    # Code block
    return value  # Optional

Example: Defining and Calling a Function

# Defining a function to greet a user
def greet(name):
    print(f"Hello, {name}!")

# Calling the function
greet("Alice")

Output:

Hello, Alice!

Explanation:

The function greet is defined to take a single parameter name and prints a greeting message.
The function is then called with the argument "Alice".

Subtopic 2: Function Parameters and Arguments

Functions can accept data via parameters. These are variables used in the function definition. The data passed into the function is called arguments.

Formal Parameters: Defined in the function header.
Actual Arguments: Passed to the function when calling it.

Example: Function with Multiple Parameters

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

# Calling the function with arguments
result = add_numbers(10, 20)
print(result)

Output:

Explanation:

The function add_numbers takes two parameters a and b, then returns their sum. It is called with arguments 10 and 20.

Subtopic 3: Default and Keyword Arguments

You can provide default values for function parameters. If the argument is not passed when calling the function, the default value is used.

Default Arguments: Arguments with predefined values.
Keyword Arguments: Arguments passed by explicitly specifying the parameter name.

Default Arguments Example

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

# Calling the function with only one argument (message will default to "Hello")
greet("Bob")

# Calling the function with both arguments
greet("Alice", "Good Morning")

Output:

Hello, Bob!
Good Morning, Alice!

Keyword Arguments Example

def greet(name, age):
    print(f"Name: {name}, Age: {age}")

# Using keyword arguments to specify the parameters
greet(age=25, name="Charlie")

Output:

Name: Charlie, Age: 25

Explanation:

In the first example, the function greet has a default value for message, so if no message is provided, it defaults to "Hello".
In the second example, keyword arguments allow you to pass arguments in any order by explicitly specifying the parameter names.

Subtopic 4: Return Values and None

Functions in Python can return values using the return keyword. If no value is returned, the function implicitly returns None.

Example: Function with Return Value

def multiply(a, b):
    return a * b

result = multiply(4, 5)
print(result)

Output:

Example: Function Returning None

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

result = print_greeting("Diana")
print(result)  # This will print None because the function does not return anything

Output:

Hello, Diana!
None

Explanation:

The multiply function returns the result of a * b, while the print_greeting function simply prints a message and does not return any value, so it implicitly returns None.

Tasks

Task 1: Function to Calculate Square of a Number
- Write a function that takes a number as a parameter and returns its square.
```
def square(num):
    return num ** 2

result = square(5)
print(result)  # Output: 25
```

Task 2: Function with Multiple Parameters

Write a function that takes two numbers and returns their sum, difference, and product.

def calculate(a, b):
    return a + b, a - b, a * b

sum, difference, product = calculate(10, 5)
print("Sum:", sum, "Difference:", difference, "Product:", product)

Task 3: Function with Default Argument
- Write a function that takes a message and a name. If no message is provided, the default message should be "Welcome". Print the message with the name.
```
def welcome(name, message="Welcome"):
    print(f"{message}, {name}!")

welcome("David")  # Uses default message
welcome("Eve", "Good Morning")  # Uses provided message
```
Task 4: Function with Keyword Arguments
- Write a function that accepts two parameters: first_name and last_name. Print a greeting message using keyword arguments.
```
def greet_person(first_name, last_name):
    print(f"Hello {first_name} {last_name}!")

greet_person(first_name="John", last_name="Doe")
```

Task 5: Function with Return Value

Write a function that returns the larger of two numbers.

def max_of_two(a, b):
    if a > b:
        return a
    return b

print(max_of_two(10, 20))  # Output: 20

Task 6: Function Returning None
- Write a function that prints "Goodbye" and does not return anything. When called, print the result to verify it returns None.
```
def say_goodbye():
    print("Goodbye!")

result = say_goodbye()
print(result)  # Output: None
```