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python_functions.md

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🐍 Created with love by Prashant

This guide introduces functions in Python — a key concept for writing clean, reusable, and modular code.

📚 What is a Function?

A function is a block of reusable code that performs a specific task.

Instead of repeating code, you define it once and call it whenever needed.

🧱 Why Use Functions?

  • Organize code better
  • Avoid repetition (DRY principle)
  • Improve readability and debugging
  • Enable reuse and testing

🛠️ Defining a Function

Basic Syntax

def function_name():
    # code block

Example

def greet():
    print("Hello, world!")

To call the function:

greet()

🧾 Functions with Parameters

Example

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

greet("Prashant")
greet("Prabesh")

You can pass multiple parameters:

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

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

🔁 Return Values

Functions can return data using the return keyword.

def square(x):
    return x * x

print(square(4))  # Output: 16

📦 Default Parameters

You can provide default values for parameters:

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

greet()         # Output: Hello, Guest
greet("Prashant")  # Output: Hello, Prashant

🧮 Keyword Arguments

Call functions using named arguments:

def intro(name, age):
    print(f"{name} is {age} years old.")

intro(age=25, name="Prabesh")

🎯 Arbitrary Arguments

Use *args and **kwargs for flexible inputs.

*args – multiple positional arguments

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

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

**kwargs – multiple keyword arguments

def display_info(**info):
    for key, value in info.items():
        print(key, ":", value)

display_info(name="Prashant", age=30)

🧠 Scope and Local Variables

Variables defined inside a function are local:

def demo():
    x = 10  # local variable / block scoped
    print(x)

demo()
# print(x)  # Error: x is not defined outside

Use global if you must access a global variable:

count = 0

def increment():
    global count
    count += 1

🔁 Recursion

A function calling itself:

def factorial(n):
    if n == 1:
        return 1
    return n * factorial(n - 1)

print(factorial(5))  # Output: 120

Use with care — make sure there's a base case to stop.

📘 Lambda (Anonymous) Functions

Short, one-line functions:

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

✅ Practice Examples

1. Function to find maximum of 3 numbers

def maximum(a, b, c):
    return max(a, b, c)

2. Function to reverse a string

def reverse_string(s):
    return s[::-1]

3. Function to check if number is even

def is_even(n):
    return n % 2 == 0

---Created with love by Prashant