Functions

Functions allow you to organize your code into reusable blocks. They help improve readability, reduce repetition, and make your code more modular.

Defining Functions

You define a function in Python using the def keyword.

Syntax:

def function_name(parameters):
    # Code to execute
    return result  # Optional

Example:

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

Calling Functions

To execute a function, you “call” it by using its name followed by parentheses.

Example:

# Define the function
def greet(name):
    print(f"Hello, {name}!")

# Call the function
greet("Alice")  # Output: Hello, Alice!

Function Parameters and Arguments

Functions can take parameters (placeholders) that are replaced with arguments (values) when the function is called.

Example:

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

result = add(5, 3)
print(result)  # Output: 8

Default Parameters:

You can provide default values for parameters.

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

greet()          # Output: Hello, Guest!
greet("Alice")   # Output: Hello, Alice!

Keyword Arguments:

You can specify arguments by name when calling the function.

def introduce(name, age):
    print(f"My name is {name} and I am {age} years old.")

introduce(age=25, name="Bob")

Return Values

Functions can return values using the return statement.

Example:

def square(num):
    return num * num

result = square(4)
print(result)  # Output: 16

If no return is used, the function returns None by default.

Scope of Variables

Variables created inside a function have local scope and are not accessible outside the function.

Example:

def my_function():
    x = 10  # Local variable
    print(x)

my_function()
# print(x)  # Error: x is not defined

*args and **kwargs

*args: Allows a function to accept any number of positional arguments.

def sum_all(*numbers):
    return sum(numbers)

print(sum_all(1, 2, 3, 4))  # Output: 10

**kwargs: Allows a function to accept any number of keyword arguments.

def print_info(**info):
    for key, value in info.items():
        print(f"{key}: {value}")

print_info(name="Alice", age=30, city="New York")

Anonymous Functions (lambda)

Lambda functions are one-liner functions defined using the lambda keyword.

Syntax:

lambda arguments: expression

Example:

square = lambda x: x * x
print(square(5))  # Output: 25

Common Use Cases

1. Reusable Logic:

   def is_even(num):
       return num % 2 == 0
   
   print(is_even(4))  # True
   print(is_even(7))  # False
   

2. Mathematical Operations:

   def add(a, b):
       return a + b
   
   print(add(3, 5))  # 8
   

3. Working with Lists:

   def double_elements(elements):
       return [x * 2 for x in elements]
   
   print(double_elements([1, 2, 3]))  # [2, 4, 6]
   

Practice Exercises

1. Write a function greet_user(name) that:
   • Takes a user’s name as input.
   • Prints a greeting like “Hello, [name]!”
2. Create a function calculate_area(length, width) that:
   • Takes two arguments (length and width).
   • Returns the area of a rectangle.
3. Define a function find_max(numbers) that:
   • Accepts a list of numbers.
   • Returns the largest number in the list.
4. Build a function is_prime(num) that:
   • Checks if a number is prime.
   • Returns True if it is, and False otherwise.

Functions are a core concept in Python and form the building blocks of reusable and efficient code!