Concept of Functions

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

Defining and Calling a Function

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

greet()      # function call
greet()      # can be called multiple times

Syntax

def function_name(parameters):
    """optional docstring"""
    statement(s)
    return value   # optional

Types of Functions

1. Built-in functions — provided by Python itself.

print(len("hello"))    # len() is built-in
print(max(3, 7, 2))     # max() is built-in

2. User-defined functions — written by the programmer.

def square(n):
    return n * n

print(square(5))   # 25

3. Lambda (anonymous) functions — small, single-expression functions defined with lambda.

square = lambda n: n * n
print(square(6))   # 36

add = lambda a, b: a + b
print(add(3, 4))    # 7

Function with no return (returns None implicitly)

def show_message():
    print("This function returns nothing")

result = show_message()
print(result)   # None

Why use functions?

• Reusability — write once, call many times.
• Modularity — break a large problem into smaller pieces.
• Readability — named functions document intent.
• Easier debugging and testing — isolate logic in small units.

Flowchart of a function call