What is a Function?

A function is a reusable block of code that does one job. Write it once, call it many times. This keeps programs short, organized and easy to fix.

Defining a Function

Use the def keyword, a name, brackets, and a colon. The indented block is the body.

def say_hello():
    print("Hello!")

say_hello()    # call it
say_hello()    # call again

Parameters & Arguments

A function can accept inputs (parameters). The values you pass in are arguments.

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

greet("Aman")
greet("Riya")

Return Values

return sends a result back to the caller so it can be stored or reused.

def square(n):
    return n * n

result = square(5)
print(result)

Program 1: Greeting Function

def greet(name, time):
    print(f"Good {time}, {name}!")

greet("Aman", "morning")
greet("Riya", "evening")

Program 2: Area Calculator

def rectangle_area(length, width):
    return length * width

def circle_area(radius):
    return 3.14 * radius * radius

print("Rectangle:", rectangle_area(10, 5))
print("Circle   :", circle_area(7))

Two small functions, each doing one calculation and returning the answer.

Program 3: Default & Keyword Arguments

def power(base, exp=2):     # exp defaults to 2
    return base ** exp

print(power(5))           # uses default exp=2
print(power(2, 3))        # exp=3
print(power(exp=4, base=2))  # keyword args

• exp=2 is a default — used if you do not pass it.
• You can pass arguments by name (keyword) in any order.

Program 4: Factorial Function

def factorial(n):
    result = 1
    for i in range(1, n + 1):
        result *= i
    return result

print("5! =", factorial(5))
print("0! =", factorial(0))

Program 5: *args — Any Number of Inputs

def total(*numbers):       # *args collects all into a tuple
    return sum(numbers)

print(total(10, 20))
print(total(1, 2, 3, 4, 5))

*numbers lets the function accept any count of values, gathered into a tuple.

Local vs Global Variables

• A variable made inside a function is local — it exists only there.
• A variable made outside is global — visible everywhere.
• To change a global from inside, use the global keyword.

Common Mistakes

• Forgetting return — the function then gives None.
• Defining a function but never calling it.
• Wrong number of arguments passed.
• Confusing print (shows) with return (gives back).

Practice Tasks

1. Write a function that returns the larger of two numbers.
2. Write a function to check if a number is prime.
3. Write a function that takes a name and an optional greeting (default "Hello").
4. Write a function using *args to find the average of any numbers.

Summary

• Functions are reusable code blocks defined with def.
• Parameters receive inputs; return sends back a result.
• Defaults, keyword args and *args make functions flexible.
• Variables inside a function are local by default.