Functions: Reusable Blocks of Code

The problem functions solve

Imagine you need to greet three people in your program:

print("Hello, Sam!")
print("Have a great day, Sam!")
print("Hello, Maya!")
print("Have a great day, Maya!")
print("Hello, Ali!")
print("Have a great day, Ali!")

That's a lot of repeated typing, and if you ever want to change the greeting, you have to fix it in many places. There's a better way: a function.

A function is a named, reusable block of code that does one job. You define it once, then call it whenever you need it.

Defining a function

In Python, you create a function with the def keyword:

def greet():
    print("Hello!")
    print("Have a great day!")

This defines a function called greet. The indented lines are the function's body — the code it runs. Defining a function does not run it yet. It just teaches Python what greet means.

Calling a function

To actually run the function, you call it by writing its name with parentheses:

greet()
greet()

This prints the greeting twice:

Hello!
Have a great day!
Hello!
Have a great day!

Write the steps once, run them as many times as you like. That's the power of functions.

Passing in information with arguments

Our greeting always says the same thing. To make it flexible, we pass in information using a parameter:

def greet(name):
    print("Hello, " + name + "!")
    print("Have a great day, " + name + "!")

Here name is a parameter — a placeholder for a value. When we call the function, we give it an argument, the real value:

greet("Sam")
greet("Maya")
greet("Ali")

Now the same three-line function handles all three people:

Hello, Sam!
Have a great day, Sam!
Hello, Maya!
...

Compare that to the repeated code at the top. Much shorter, and changing the greeting now means editing just one place.

Returning a value

Some functions don't just do something — they calculate a result and hand it back. For that we use return:

def double(n):
    return n * 2

The return keyword sends a value back out of the function. We can store it in a variable or print it:

result = double(5)
print(result)      # prints 10
print(double(7))   # prints 14

When Python reaches return, it stops the function and sends the value back to wherever the function was called. A function can use that result in math, in an if statement, or anywhere a value fits.

Functions and loops together

Functions and loops are a powerful team. A loop can call a function many times:

def square(n):
    return n * n

for i in range(1, 6):
    print(i, "squared is", square(i))

This prints:

1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25

The loop handles the repetition; the function handles the calculation. Each part does one clear job.

Why functions matter

Functions are a cornerstone of good programming because they let you:

• Reuse code instead of copying it (programmers call this "Don't Repeat Yourself").
• Organize a big program into small, named pieces, each doing one job.
• Fix bugs faster — when a job is wrong, you correct it in one place. Pair this with careful debugging.
• Hide detail — once square works, you can use it without thinking about how it works inside.

This idea scales all the way up to professional software and artificial intelligence, where huge programs are built from thousands of small functions working together. Master functions, and you can build programs of almost any size.