Defining and Calling Functions

Why this matters

You have already used functions: print, len, input. Now you make your own. A function is a named block of code you can run whenever you like. You write it once and use it many times, which means fewer bugs and shorter programs.

The idea

def greet(name):
    print(f"Hello, {name}!")

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

• def starts the definition. greet is the function’s name.
• name is a parameter: a variable that exists inside the function and receives a value each time it is called.
• The indented lines are the body. They run only when the function is called, with greet("Ada"). The value "Ada" is the argument.

Defining a function does not run it. Nothing prints until you call it.

A function can have several parameters, separated by commas. Arguments are matched up in order:

def add_and_show(a, b):
    print(a + b)

add_and_show(2, 3)   # 5

Try it

Define shout(word) that prints the word in capitals followed by !. Call it three times with different words.

Your turn

Define two functions in the starter. The checks call them for you (do not call them yourself):

1. greet(name) prints Hello, NAME!
2. banner(text) prints the text in a box, with a border of = that is 4 longer than the text:

=========
| Hello |
=========

Recap

• def name(parameters): followed by an indented body defines a function.
• Calling name(arguments) runs the body with the parameters filled in.
• Defining is not calling: the code inside runs only when called.

Common mistakes

• Forgetting the colon after the parentheses: SyntaxError.
• Forgetting to indent the body: IndentationError.
• Defining but never calling, so nothing happens. (Here, the checks do the calling.)
• Wrong number of arguments: TypeError: greet() takes 1 positional argument but 2 were given.