Defining and Calling Functions

A function is a named, reusable block

By now some of your programs repeat the same few lines with tiny changes, and repetition is where bugs breed: fix one copy, forget the other. Functions exist to kill that duplication: define the step once, name it, reuse it everywhere, fix it in one place. Every library you will ever import is a pile of functions somebody defined, so this lesson is also how you will read other people's code.

You have been calling functions all course: print(), len(), input(). Now you get to define your own with def:

def area(w, h):
    return w * h

• area is the function's name.
• w and h are parameters: placeholder variables for the inputs.
• return sends a value back to whoever called.

Defining runs nothing. The body executes only when you call it, and the values you pass, the arguments, get assigned to the parameters:

print(area(3, 4))    # w becomes 3, h becomes 4, back comes 12

Write a function whenever the same few lines would otherwise be pasted twice, or when naming a step (area, greet) makes the program read like a plan.

Default values and keyword arguments

A parameter can carry a default value, written with = in the def line. The default is used whenever the caller leaves that argument out. Defaults exist so the common call stays short while the unusual call stays possible:

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

greet("Ada")             # Hello, Ada!
greet("Ada", "Welcome")  # Welcome, Ada!

You can also name arguments at the call site, called keyword arguments: greet(greeting="Hi", name="Grace"). With a keyword argument the name, not the position, decides which parameter receives the value. Real code uses this to keep calls readable, and print itself accepts one: print("a", "b", sep="-") prints a-b, where sep replaces print's default separator, a space.