Welcome back to the Python Tutorial Series for Beginners! 🚀

In Lesson 10, we learned about loops and how they help us repeat tasks efficiently.

Now in Lesson 11, we’ll dive into functions – the building blocks of organized and reusable code.

By the end of this lesson, you’ll understand:

• What functions are and why they’re important
• How to define and call a function
• Function parameters and return values
• Default arguments and keyword arguments
• Variable scope in functions
• Lambda (anonymous) functions
• Real-world use cases
• Beginner exercises

🔹 What are Functions?

👉 A function is a block of code that performs a specific task and can be reused whenever needed.

💡 Real-life example:

• A vending machine: You press a button (call function) → it dispenses a drink (returns output).

In Python, we use the def keyword to create functions.

🔹 1. Defining and Calling a Function

def greet():
   print("Hello, welcome to Python!")
   
# Calling the function
greet()

👉 Output:

Hello, welcome to Python!

🔹 2. Function Parameters

Functions can accept parameters (inputs).

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

👉 Output:

Hello, Alice!
Hello, Bob!

🔹 3. Return Values

Functions can also return results using the return keyword.

def add(a, b):
   return a + b
   
result = add(5, 3)
print("Sum =", result)

👉 Output: Sum = 8

🔹 4. Default Arguments

You can assign default values to parameters.

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

👉 Output:

Hello, Guest!
Hello, Charlie!

🔹 5. Keyword Arguments

You can specify arguments by name for clarity.

def introduce(name, age):
   print(f"My name is {name}, and I am {age} years old.")
   
introduce(age=25, name="David")

👉 Output:

My name is David, and I am 25 years old.

🔹 6. Variable Scope

Variables inside a function are local and cannot be accessed outside.

def my_function():
   x = 10  # local variable
   print("Inside function:", x)
   
my_function()
print("Outside function:", x)  # ❌ Error

👉 Output:

Inside function: 10
NameError: name 'x' is not defined

🔹 7. Lambda (Anonymous Functions)

Lambda functions are small, one-line functions.

square = lambda x: x * x
print(square(5))

👉 Output: 25

Useful for short operations like sorting, filtering, or quick calculations.

🔹 8. Real-World Use Cases

✅ Calculator Function

def calculator(a, b, operation):
   if operation == "add":
       return a + b
   elif operation == "subtract":
       return a - b
   elif operation == "multiply":
       return a * b
   elif operation == "divide":
       return a / b
   else:
       return "Invalid operation"
       
print(calculator(10, 5, "add"))
print(calculator(10, 5, "multiply"))

✅ Reusable Greeting Function

def welcome_user(username):
   print(f"Welcome back, {username}!")

✅ Filtering with Lambda

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = list(filter(lambda x: x % 2 == 0, numbers))
print(even_numbers)

👉 Output: [2, 4, 6]

🔹 Exercises for Beginners

Try these in a new file (lesson11.py):

1. Write a function that prints “Good Morning” when called.
2. Create a function that takes two numbers and returns their product.
3. Write a function that checks if a number is even or odd.
4. Create a function with a default parameter for age (e.g., 18).
5. Challenge: Write a function that takes a list of numbers and returns the maximum number.

🎯 Recap

In this lesson, you learned:
✅ Functions help organize and reuse code
✅ Parameters and return values make functions flexible
✅ Default and keyword arguments give more control
✅ Variable scope prevents accidental conflicts
✅ Lambda functions provide quick one-liners
✅ Functions are essential in real-world programming

Next up: Lesson 12 – Modules and Packages in Python 📦 (where we learn how to organize large projects).