Function Parameters — Passing Data In

By the end of this lesson you can:

• Add a parameter to a function with def greet(name): and use the name inside the body.
• Pass an argument when you call: greet("Aisyah") vs greet("Faiz"), and see the function behave differently.
• Tell the difference between a parameter (in the def) and an argument (in the call) without flinching.

You've already been passing inputs to functions for ages, you just haven't noticed. Look:

print("Hello, world!")
len("Selamat datang")
sorted("nasi")

print, len, sorted — all functions, all of them take an input inside the parentheses. The input you put inside is called an argument. The fact that print behaves differently each call (printing different text) is exactly because you handed it a different argument.

Today you learn how to build a function of your own that accepts an argument the same way.

Define a function with a parameter

Put a name inside the parentheses of the def line. That name becomes a temporary variable that exists only inside the function body:

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

Now greet is a recipe with a blank: "print Hello, then whatever you handed me, then a !". The blank is called name — that's the parameter.

Pass an argument when you call

To fill in the blank, put a value inside the parentheses of the call:

greet("Aisyah")
greet("Faiz")
greet("Mei")

Hello, Aisyah!
Hello, Faiz!
Hello, Mei!

Three calls, three different arguments, three different greetings. The function body is written exactly once, but it produces different output for every call.

Inside the function, the parameter is just a variable

You can use the parameter like any other variable — store it, slice it, chain methods on it (callback from PY-L1-23 and PY-L1-24):

def shout_initial(name):
    first_letter = name[0].upper()
    print("Your initial is", first_letter)

shout_initial("aisyah")
shout_initial("faiz")

Your initial is A
Your initial is F

Forgetting the argument — TypeError

If the function expects an argument and you forget to pass one, Python tells you off:

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

greet()   # ← oops, no argument

TypeError: greet() missing 1 required positional argument: 'name'

Read the error: "missing 1 required positional argument: 'name'". Python is even telling you which parameter you forgot. Fix it by passing a value inside the parentheses.

The parameter only exists inside the function

This is the most common confusion. name in the function body is not the same as a name variable in the main program. The function's parameter is born when the call starts and dies when the call ends:

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

greet("Aisyah")
print(name)   # ← NameError. There is no 'name' out here.

The fact that the slot is called name is a private detail of the function. From the outside, all that matters is "greet takes one argument".

Step 1 · Add a slot

Take last lesson's welcome_customer and give it a name parameter:

def welcome_customer(name):
    print("=" * 35)
    print("Selamat datang,", name + "!")
    print("=" * 35)

Step 2 · Call it with different arguments

welcome_customer("Aisyah")
welcome_customer("Faiz")
welcome_customer("Mei")

===================================
Selamat datang, Aisyah!
===================================
===================================
Selamat datang, Faiz!
===================================
===================================
Selamat datang, Mei!
===================================

Step 3 · Combine with input()

Arguments don't have to be hard-coded strings. They can be anything that evaluates to a value — including the result of input():

Code

# cafe.py — personalised greeter

def welcome_customer(name):
    print("=" * 35)
    print("Selamat datang,", name + "!")
    print("=" * 35)
    print("Tonight's menu: roti canai, nasi lemak, teh tarik.")
    print()

# main program
first = input("First customer's name: ").strip()
welcome_customer(first)
order1 = input("Order: ")

second = input("Second customer's name: ").strip()
welcome_customer(second)
order2 = input("Order: ")

print()
print(first, "ordered", order1)
print(second, "ordered", order2)

Sample run

First customer's name: Aisyah
===================================
Selamat datang, Aisyah!
===================================
Tonight's menu: roti canai, nasi lemak, teh tarik.

Order: roti canai
Second customer's name: Faiz
===================================
Selamat datang, Faiz!
===================================
Tonight's menu: roti canai, nasi lemak, teh tarik.

Order: nasi lemak

Aisyah ordered roti canai
Faiz ordered nasi lemak

One function, one body, two completely different welcome screens. And the main program at the bottom still reads top-to-bottom like a story.

Three tasks. Define each function once; call it with several different arguments.

def greet(name):
    print("Hi,", name + "! Nice to meet you.")

greet("Aisyah")
greet("Faiz")
greet("Mei")

def print_squared(n):
    print(n, "squared is", n * n)

print_squared(3)
print_squared(7)
print_squared(12)

def echo(word):
    print(word)
    print(word)
    print(word)

raw = input("Type a word: ")
echo(raw)

A function becomes truly useful when it can accept input from the caller. The parameter is the named slot in the def line — def greet(name):. The argument is the value you pass at the call site — greet("Aisyah"). Inside the body, the parameter behaves like a normal variable, but it only exists for the duration of that one call. Forget the argument and Python raises TypeError with a helpful message. Next lesson we'll add a second parameter — and a way to give it a sensible default value when the caller doesn't feel like passing one.