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Build Your Own Commands

Understand how to define and use functions with arguments.

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We’ve been using built-in functions like print() and input(). Now it’s our turn to create our reusable commands using functions.

Think of a function like a recipe:

  • It has a name.

  • It can take ingredients (inputs).

  • It does something when you use it.

Built-in vs. user-defined functions

  • Built-in functions are already provided by Python, like print() or len().

  • User-defined functions are ones you create using the def keyword.

Creating your functions lets you:

  • Reuse your code.

  • Organize your program.

  • Make it easier to read and understand.

Writing our first function

You’re about to create a simple function that says hello.

Python
def greet():        # Define a new function
    print("Hello!") # What it does
greet()             # Call the function

We just created and called our first custom function!

What’s happening here?

  • def starts a function definition.

  • greet is the function name.

  • () means it can take inputs (called parameters).

  • greet() runs the function.

Adding your own input

Now let’s make your function flexible by passing in a name.

Python
def greet(name):
    print("Hello,", name)
greet("Ava")
greet("Zane")

Now the function works with any name we give it!

Returning a value

Some functions give something back using the return keyword.

Python
def add(a, b):
    return a + b
result = add(5, 3)
print(result)

The return keyword sends a value back. We can store or use it.

Functions can do work and return a value you can use later.

Try this: Create your own fun function

Create your own message function!

Python
def favorite_color(color):
    print("My favorite color is", color)
favorite_color("green")
favorite_color("purple")

Try changing the message or the function name to make it your own!