Lesson 7: Functions in Python

🎯 Lesson Objective

To understand what functions are, how to create and use them, and why they are essential for code organization and reuse.

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🧠 1. What Is a Function?

A function is a reusable block of code that performs a specific task.
Functions make programs easier to read, test, and maintain.

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⚙️ 2. Syntax of a Function

def function_name(parameters):
    # code block
    return value

✅ Example:

def greet():
    print("Hello, welcome to Python!")

greet()

Output:

Hello, welcome to Python!

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🧩 3. Function with Parameters

Functions can accept inputs (called parameters) that make them flexible.

def add_numbers(a, b):
    result = a + b
    print("Sum:", result)

add_numbers(5, 7)

Output:

Sum: 12

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🧾 4. Function with Return Statement

A function can return data back to the caller using the return keyword.

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

result = multiply(4, 6)
print("Product:", result)

Output:

Product: 24

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🔁 5. Default Parameters

If a parameter value isn’t provided, Python uses the default.

def greet(name="User"):
    print("Hello,", name)

greet("Sameer")
greet()

Output:

Hello, Sameer
Hello, User

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🧮 6. Function with Multiple Returns

A function can return multiple values separated by commas.

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

sum_val, diff_val = calculate(10, 5)
print("Sum:", sum_val)
print("Difference:", diff_val)

Output:

Sum: 15
Difference: 5

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🔄 7. Variable Scope

Variables defined inside a function are local, while those outside are global.

x = 10  # Global variable

def show():
    x = 5  # Local variable
    print("Inside function:", x)

show()
print("Outside function:", x)

Output:

Inside function: 5
Outside function: 10

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🧰 8. Lambda (Anonymous) Functions

A lambda function is a short, one-line function without a name.

square = lambda x: x * x
print(square(4))

Output:

16

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🌍 9. Real-Life Example: 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, "divide"))

Output:

15
2.0

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🧠 10. Practice Ideas

1. Write a function to find the factorial of a number.
2. Create a function that checks whether a number is even or odd.
3. Build a function to count vowels in a string.
4. Write a function to convert Celsius to Fahrenheit.
5. Make a calculator using functions for each operation.
6. Use a lambda function to find the square and cube of a number.