Functions Explained for Beginners

A function is a set of steps you give a name, so you can run those steps whenever you want without writing them out again.

def greet(name):
    return "Hello, " + name + "!"

print(greet("Sam"))
print(greet("Ada"))

Written once, used twice. Change the greeting in one place and both calls update.

The three parts of a function#

• A name — what it does, usually a verb: calculate_total, send_email.
• Inputs (parameters) — the information it needs, in brackets.
• An output (return value) — the answer it hands back.

def area(width, height):
    return width * height

print(area(3, 4))    # 12

width and height are parameters — placeholder names. The 3 and 4 you pass in are arguments — the real values.

function area(width, height) {
  return width * height;
}

console.log(area(3, 4));   // 12

Return versus print — the one that confuses everyone#

print shows something to a person. return hands a value back to the code that called the function. They are not interchangeable.

def double_printed(n):
    print(n * 2)

def double_returned(n):
    return n * 2

result = double_printed(5)    # displays 10, result is None
result = double_returned(5)   # displays nothing, result is 10
print(result * 3)             # 30 — only works with the second one

If a function’s answer needs to be used in further calculations, it must return. A function with no return hands back “nothing” (None in Python, undefined in JavaScript), which is why None * 3 errors.

Default values#

def greet(name, greeting="Hello"):
    return greeting + ", " + name + "!"

print(greet("Sam"))              # Hello, Sam!
print(greet("Sam", "Welcome"))   # Welcome, Sam!

Variables inside a function stay inside it#

A variable created inside a function does not exist outside it. This is called scope, and it is a feature rather than an inconvenience — two functions can both use a variable called total without interfering with each other.

def calculate():
    subtotal = 100
    return subtotal

calculate()
print(subtotal)   # NameError: name 'subtotal' is not defined

If you need the value outside, return it and store the result.

When should something become a function?#

1. You copied and pasted code. The second copy is the moment to make a function.
2. You want to name a step. If a block needs a comment explaining what it does, that comment is usually the function name.
3. A block does more than one job. “Fetch the data and format it and save it” is three functions wearing a trench coat.

You do not need to make everything a function. A five-line script is fine as it is.

A worked example#

def apply_discount(price, percent):
    return price - (price * percent / 100)

def format_money(amount):
    return "Rs " + str(round(amount, 2))

original = 2500
final = apply_discount(original, 15)

print("Was:", format_money(original))
print("Now:", format_money(final))

Two small functions, each doing one thing, combined at the end. That is the shape most well-organised programs take.

Where to go next#

• Arrays and lists explained
• Python functions in more depth
• JavaScript functions and arrow syntax