A function is a reusable block of code that performs a specific task. Python has built-in functions, user-defined functions, lambda functions, and more.

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1️⃣ Defining a Function

A function is defined using the def keyword.

✅ Example:

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

greet()  # Calling the function

Output:

Hello, Welcome to Python!

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2️⃣ Function with Parameters and Return Value

Functions can take parameters (inputs) and return values.

✅ Example:

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

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

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3️⃣ Types of Functions in Python

Python provides various types of functions:

Function Type	Description	Example
Built-in Functions	Predefined in Python	print(), len(), max()
User-defined Functions	Created by the user	def my_function():
Lambda Functions	Anonymous (one-liner) functions	lambda x: x * 2
Recursive Functions	A function that calls itself	Factorial calculation
Higher-order Functions	Functions that take other functions as arguments	map(), filter(), reduce()

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4️⃣ Built-in Functions

Python has many built-in functions, such as:

Function	Purpose
len(seq)	Returns length of a sequence
sum(iterable)	Returns sum of elements
max(iterable)	Returns max element
min(iterable)	Returns min element
sorted(iterable)	Returns sorted sequence
round(number, ndigits)	Rounds a number
abs(x)	Returns absolute value

✅ Example:

numbers = [4, 1, 8, 3]
print(max(numbers))  # Output: 8
print(sum(numbers))  # Output: 16
print(sorted(numbers))  # Output: [1, 3, 4, 8]

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5️⃣ Lambda Functions (Anonymous Functions)

A lambda function is a one-liner function without a name.

✅ Example:

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

✅ Example with map() function:

numbers = [1, 2, 3, 4]
squared_numbers = list(map(lambda x: x * x, numbers))
print(squared_numbers)  # Output: [1, 4, 9, 16]

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6️⃣ Recursive Functions

A recursive function calls itself to solve a problem.

✅ Example (Factorial Calculation):

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

print(factorial(5))  # Output: 120

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7️⃣ Higher-order Functions (map(), filter(), reduce())

Higher-order functions take other functions as arguments.

✅ Using map() (applies a function to each element):

numbers = [1, 2, 3, 4]
squared = list(map(lambda x: x ** 2, numbers))
print(squared)  # Output: [1, 4, 9, 16]

✅ Using filter() (filters elements based on condition):

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = list(filter(lambda x: x % 2 == 0, numbers))
print(even_numbers)  # Output: [2, 4, 6]

✅ Using reduce() (reduces a list to a single value):

from functools import reduce

numbers = [1, 2, 3, 4]
product = reduce(lambda x, y: x * y, numbers)
print(product)  # Output: 24

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🔹 Summary Table

Function Type	Description
Built-in Functions	Predefined in Python (print(), max())
User-defined Functions	Created using def
Lambda Functions	One-liner anonymous functions
Recursive Functions	A function that calls itself
Higher-order Functions	Functions that accept other functions (map(), filter(), reduce())

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