• Function = reusable block of code.
• Defined using def keyword.
• Helps reduce repetition and improves readability.

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

greet()

• Arguments pass data to a function.
• Common types: positional, keyword, default, variable-length.

# Positional arguments
def add(a, b):
    print(a + b)

add(10, 20)

# Keyword arguments
add(b=5, a=2)

# Default arguments
def welcome(name="Guest"):
    print("Welcome", name)

welcome()
welcome("Sourav")

# Variable-length arguments (*args)
def total_sum(*nums):
    print(sum(nums))

total_sum(1, 2, 3, 4)

• return sends value back to the caller.
• Function ends immediately after return.
• Can return multiple values (as tuple).

def square(n):
    return n * n

ans = square(5)
print(ans)

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

addv, subv = calc(10, 3)
print(addv, subv)

• Short function written in one line using lambda.
• Used for quick operations (often with map/filter/sort).

square = lambda x: x * x
print(square(6))

add = lambda a, b: a + b
print(add(10, 20))

nums = [1, 2, 3, 4]
even = list(filter(lambda x: x % 2 == 0, nums))
print(even)

• Recursion = function calls itself.
• Must have a base condition to stop recursion.
• Used in factorial, Fibonacci, tree problems, etc.

def factorial(n):
    if n == 0 or n == 1:
        return 1
    return n * factorial(n - 1)

print(factorial(5))   # 120

• Local: defined inside a function (only usable inside).
• Global: defined outside function (usable everywhere).
• Use global keyword to modify global variable inside function.

x = 10   # global

def show():
    x = 5    # local
    print("Inside:", x)

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

def change():
    global x
    x = 99

change()
print("After change:", x)

• Module = a Python file with functions, variables, classes.
• Used to organize code and reuse functionality.
• Use import to include a module.

import math

print(math.sqrt(25))
print(math.pi)

from math import factorial
print(factorial(5))

import math as m
print(m.pow(2, 3))

• math → math operations
• random → random numbers
• datetime → date & time
• os → operating system tasks
• sys → system-specific parameters

import random
import datetime

print(random.randint(1, 10))
print(datetime.datetime.now())

• Create your own module as a .py file.
• Import it like built-in modules.

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

def sub(a, b):
    return a - b

import mymath

print(mymath.add(10, 5))
print(mymath.sub(10, 5))