LEARN COMPLETE PYTHON IN 24 HOURS

12. Functions – Reusing Code

A function is a block of code that performs a specific task. You define it once and call it many times — avoids repetition (DRY principle: Don't Repeat Yourself).

12.1 Defining & Calling Functions

Syntax to define a function:

Python

def function_name(parameters): # optional parameters # code to execute # optional return statement

Calling (using) the function:

Python

function_name(arguments)

Example – Simple function

Python

def greet(): print("Hello, Anshuman!") print("Welcome to Python 2026!") # Call the function greet() greet() # You can call it multiple times

Output:

text

Hello, Anshuman! Welcome to Python 2026! Hello, Anshuman! Welcome to Python 2026!

Example with parameter

Python

def greet(name): print(f"Hello, {name}! How are you?") greet("Anshuman") greet("Rahul")

Output:

text

Hello, Anshuman! How are you? Hello, Rahul! How are you?

Tip: Function name should be lowercase with underscores (snake_case) — e.g., calculate_area

12.2 Parameters vs Arguments

• Parameter: Variable listed inside the parentheses in the function definition (placeholder)

• Argument: Actual value passed when you call the function

Python

def add(a, b): # a, b are parameters print(a + b) add(5, 3) # 5, 3 are arguments add(10, 20) # positional arguments

Output:

text

8 30

12.3 Default Arguments

You can give parameters default values — they become optional.

Syntax:

Python

def function_name(param=default_value): ...

Example:

Python

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

Important Rule: Default parameters must come after non-default ones

Python

# Correct def info(name, age=25, city="Muzaffarpur"): print(name, age, city) # Wrong → SyntaxError # def info(age=25, name, city): ...

12.4 Keyword Arguments

Pass arguments using parameter names — order doesn't matter.

Example:

Python

def person_info(name, age, city): print(f"{name} is {age} years old from {city}") person_info(age=25, name="Anshuman", city="Muzaffarpur") # Order doesn't matter when using keywords

Mix positional + keyword (positional first)

Python

person_info("Rahul", city="Patna", age=24)

Tip: Keyword arguments make code more readable when there are many parameters.

12.5 args and *kwargs

Used when you don't know how many arguments will be passed.

• *args → collects extra positional arguments into a tuple

• **kwargs → collects extra keyword arguments into a dictionary

*Example – args

Python

def sum_numbers(*args): total = 0 for num in args: total += num return total print(sum_numbers(1, 2, 3)) # 6 print(sum_numbers(10, 20, 30, 40)) # 100 print(sum_numbers()) # 0

**Example – kwargs

Python

def print_info(**kwargs): for key, value in kwargs.items(): print(f"{key}: {value}") print_info(name="Anshuman", age=25, city="Muzaffarpur", hobby="coding")

Output:

text

name: Anshuman age: 25 city: Muzaffarpur hobby: coding

Combined Example

Python

def advanced_func(a, b, args, *kwargs): print("a:", a) print("b:", b) print("args:", args) print("kwargs:", kwargs) advanced_func(10, 20, 30, 40, name="Test", city="Bihar")

Output:

text

a: 10 b: 20 args: (30, 40) kwargs: {'name': 'Test', 'city': 'Bihar'}

Tip: Common in real libraries (e.g., print(), matplotlib.plot())

12.6 Return Statement

return sends a value back to the caller. Function stops after return.

Examples:

Python

def square(num): return num * num result = square(5) print(result) # 25 print(square(10)) # 100

Multiple returns (one at a time)

Python

def check_even_odd(num): if num % 2 == 0: return "Even" else: return "Odd" print(check_even_odd(10)) # Even

Return multiple values (packs into tuple)

Python

def get_person(): return "Anshuman", 25, "Muzaffarpur" name, age, city = get_person() # unpacking print(name, age, city)

Tip: If no return, function returns None by default.

12.7 Scope – Local vs Global Variables

Local scope → Variables defined inside function (only accessible inside) Global scope → Variables defined outside (accessible everywhere, but careful)

Example:

Python

x = 100 # global def my_func(): y = 200 # local print("Inside function: x =", x) # can read global print("y =", y) my_func() print("Outside: x =", x) # print(y) # Error! y is local

Modify global inside function

Python

count = 0 def increment(): global count count += 1 print("Count:", count) increment() # Count: 1 increment() # Count: 2 print(count) # 2

Tip: Avoid modifying globals too much — pass as parameters instead for cleaner code.

12.8 Lambda Functions (Anonymous Functions)

Short, nameless functions — used for one-liners.

Syntax:

Python

lambda arguments: expression

Examples:

Python

# Normal function def square(x): return x x # Lambda equivalent square_lambda = lambda x: x x print(square_lambda(5)) # 25

Common uses – with map(), filter(), sort()

Python

numbers = [1, 2, 3, 4, 5] # Double each doubled = list(map(lambda x: x * 2, numbers)) print(doubled) # [2, 4, 6, 8, 10] # Filter even evens = list(filter(lambda x: x % 2 == 0, numbers)) print(evens) # [2, 4] # Sort by length words = ["apple", "banana", "kiwi"] sorted_words = sorted(words, key=lambda w: len(w)) print(sorted_words) # ['kiwi', 'apple', 'banana']

Mini Project – Calculator using Functions

Python

def add(x, y): return x + y def subtract(x, y): return x - y def multiply(x, y): return x y def divide(x, y): if y == 0: return "Cannot divide by zero!" return x / y operations = { "+": add, "-": subtract, "": multiply, "/": divide } print("Simple Calculator") num1 = float(input("First number: ")) op = input("Operator (+ - * /): ") num2 = float(input("Second number: ")) if op in operations: result = operations[op](num1, num2) print(f"{num1} {op} {num2} = {result}") else: print("Invalid operator!")

This completes the full Functions section — now you can write reusable, powerful code!

📚 Amazon Book Library

All my books are FREE on Amazon Kindle Unlimited🌍 Exclusive Country-Wise Amazon Book Library – Only Here!

On GlobalCodeMaster.com you’ll find complete, ready-to-use lists of my books with direct Amazon links for every country.

Belong to India, Australia, USA, UK, Canada or any other country? Just click your country’s link and enjoy:

✅ Any eBook FREE on Kindle Unlimited ✅ Or buy at incredibly low prices

400+ fresh books written in 2025-2026 with today’s latest AI, Python, Machine Learning & tech trends – nowhere else will you find this complete country-wise collection on one platform!

Choose your country below and start reading instantly 🚀

BOOK LIBRARY USA 2026 LINK

BOOK LIBRARY INDIA 2026 LINK

BOOK LIBRARY AUSTRALIA 2026 LINK

BOOK LIBRARY CANADA 2026 LINK

BOOK LIBRARY UNITED KINGDOM 2026 LINK

BOOK LIBRARY GERMANY 2026 LINK

BOOK LIBRARY FRANCE 2026 LINK

BOOK LIBRARY ITALY 2026 LINK

BOOK LIBRARY SPAIN 2026 LINK

BOOK LIBRARY NETHERLANDS 2026 LINK

BOOK LIBRARY BRAZIL 2026 LINK

BOOK LIBRARY MEXICO 2026 LINK

BOOK LIBRARY JAPAN 2026 LINK

BOOK LIBRARY POLAND 2026 LINK

BOOK LIBRARY IRELAND 2026 LINK

BOOK LIBRARY SWEDEN 2026 LINK

BOOK LIBRARY BELGIUM 2026 LINK