Python Chapter 6 - Functions

Chapter 6: Functions in Python

Function ka matlab hota hai — ek reusable block of code jo baar-baar call kiya ja sakta hai. Isme hum arguments de sakte hain, return value le sakte hain, aur code ko modular aur readable bana sakte hain.

Function Define Karna:

    def greet():

      print("Hello!")

Hello!

Function Call Karna:

greet()

Function with Parameters:

    def greet(name):

      print("Hello", name)

    greet("Ravi")

Hello Ravi

Return Value:

    def add(a, b):

      return a + b

    result = add(2, 3)

    print(result)

General Mistakes:

    def add(a, b):

      a + b   # ❌ Return nahi diya

    print(add(2, 3))  # ❌ Output: None

Recursion in Python

Recursion matlab ek function khud ko call karta hai jab tak ek condition satisfy na ho jaaye. Isko step-by-step solve kiya jaata hai.

Structure of Recursive Function:

    def function_name():

      if base_condition:

        return result

      else:

        return function_name()

Example: Countdown

    def countdown(n):

      if n == 0:

        print("Liftoff!")

      else:

        print(n)

        countdown(n - 1)

    countdown(5)

5
4
3
2
1
Liftoff!

Rules of Recursion:

Base case zaroori hota hai – ye stopping point hai.
Har recursive call input ko chhota banata hai.
Python me ~1000 depth tak hi jaa sakte ho.

Common Mistake:

    def bad(n):

      return bad(n-1)  # ❌ No base case

    bad(5)  # ❌ RecursionError: maximum depth exceeded

Example: Factorial

    def factorial(n):

      if n == 1:

        return 1

      else:

        return n * factorial(n-1)

    print(factorial(5))

120

Advanced Example: Fibonacci

    def fib(n):

      if n <= 1:

        return n

      return fib(n-1) + fib(n-2)

    print(fib(6))

Lambda Functions

Lambda function ek short anonymous function hoti hai, jisme sirf ek expression hota hai. Mostly quick calculations, filter/map me use hoti hai.

Syntax:

lambda arguments: expression

Simple Example:

    double = lambda x: x * 2

    print(double(4))

With map():

    nums = [1, 2, 3, 4]

    squares = list(map(lambda x: x*x, nums))

    print(squares)

[1, 4, 9, 16]

With filter():

    nums = [1, 2, 3, 4, 5]

    even = list(filter(lambda x: x % 2 == 0, nums))

    print(even)

[2, 4]

With sorted():

    students = [('A', 50), ('B', 70), ('C', 40)]

    students.sort(key=lambda x: x[1])

    print(students)

[('C', 40), ('A', 50), ('B', 70)]

Common Mistake:

    lambda x, y: x + y  # ❌ Not used or assigned

    # ✔️ Correct:

    add = lambda x, y: x + y

    print(add(3, 4))

Lambda inside Function:

    def multiplier(n):

      return lambda x: x * n

    times3 = multiplier(3)

    print(times3(5))

Lambda vs def:

Lambda: One-line, quick use, anonymous
def: Multi-line, named, complex logic ke liye