Defining and Calling Functions

Defining and Calling Functions

In this tutorial, we'll create functions with def and run them by calling their names.

To create a function, use def.

def say_hello():
    print("Hello")

This defines the function. It does not run it yet.

To run the function, call it:

say_hello()

Output:

Hello

The Shape of a Function

A function definition has:

• The word def
• The function name
• Parentheses
• A colon
• An indented block of code

def function_name():
    code_to_run

Indentation matters. The indented lines belong to the function.

Defining vs Calling

This code only defines the function:

def greet():
    print("Hi")

Nothing appears on screen because the function was not called.

This code defines and calls it:

def greet():
    print("Hi")

greet()

Calling More Than Once

Functions can be called many times:

def show_menu():
    print("1. Start")
    print("2. Settings")
    print("3. Quit")

show_menu()
show_menu()

The same block runs each time.

Common Mistake

Forgetting parentheses means you refer to the function, but do not call it:

greet    # does not run the function
greet()  # runs the function

Define Before You Call

Python reads files from top to bottom. Define the function before calling it:

def say_goodbye():
    print("Goodbye")

say_goodbye()

If you call say_goodbye() before the def line has run, Python will not know the function exists yet.

Practice

Create a function called show_profile() that prints a name, favourite language, and current goal. Call it twice.