Functions — reusable building blocks

Imagine writing the same calculation in four different places in your program. When you discover a bug in it, you have to find and fix it four times — and hope you do not miss one. Functions solve this by giving a name to a piece of logic that you write once and call as many times as you need.

There is a deeper reason too: functions let you give names to concepts, not just values. A function called calculate_tax tells you what it does before you read a single line of its body. Good functions make code read like a description of intent, not a transcript of operations.

Defining a function with def

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

greet("Ada")      # Hello, Ada
greet("Grace")    # Hello, Grace

• def starts the definition.
• greet is the function's name.
• name inside the parentheses is a parameter — a local variable that will hold whatever value the caller passes in.
• greet("Ada") is a call; "Ada" is the argument — the actual value.

Returning a value

A function that just prints is limited — you cannot use its result in a larger calculation. return hands a value back to the caller so it can be stored, printed, or passed on:

def add(a, b):
    return a + b

result = add(3, 4)   # result is 7
print(result * 2)    # 14

Parameters and arguments

The terms are related but distinct:

• Parameter — the name in the function definition, def greet(name):
• Argument — the actual value passed when calling, greet("Ada")

A function can take any number of parameters, separated by commas:

def describe(city, country, population):
    return f"{city}, {country} has a population of {population:,}"

print(describe("Tokyo", "Japan", 13_960_000))

Variables inside functions: scope

Variables created inside a function exist only while that function is running. This is called scope — each function has its own private workspace that disappears when the function returns. This means you can name a variable total inside ten different functions without any of them interfering with each other.

def double(x):
    result = x * 2   # 'result' exists only inside this function
    return result

print(double(5))     # 10
print(result)        # NameError: result is not defined here

Exercise

Where to go next

Next: lists — storing multiple values together in a single collection.