Functions: Reusable Blocks of Code

Packaging Code into Functions

A function is a named block of code that performs one job. You write it once and then call it whenever you need it. Functions keep programs short, organised, and easy to fix.

Defining a Function

Use the def keyword, give the function a name, and add parentheses. The indented lines below form the function body.

def greet():
    print("Hello from DCCPS!")

greet()   # this line calls the function

Defining a function does not run it. Nothing happens until you call it by writing its name followed by parentheses.

Parameters and Arguments

A parameter is an input listed when you define the function. An argument is the real value you pass when you call it.

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

greet("Ayesha")   # name becomes "Ayesha"
greet("Rahim")    # name becomes "Rahim"

Returning a Value

The return keyword sends a result back to whoever called the function. The caller can store that result in a variable.

def add(x, y):
    return x + y

result = add(8, 5)
print(result)        # 13
print(add(100, 1))   # 101

There is a difference between print and return. print only displays text on screen. return hands back a value your program can keep using.

Default Parameter Values

You can give a parameter a default so the argument becomes optional.

def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25  uses default exponent 2
print(power(5, 3))   # 125 exponent given as 3

Why Functions Matter

• They remove repetition — write the logic once.
• They make code readable — a good name explains intent.
• They make bugs easy to fix — correct one place, not many.

A function should do one clear thing. If you struggle to name it, it may be doing too much — split it into smaller functions.