Week Four

This week we learn about functions — named blocks of code that perform a task and can be reused throughout your program.

Presentation

Functions

A function is a named block of code that performs a task. Functions help you organize your code and reuse code instead of writing the same instructions again.

Function Syntax

Functions in Python follow a specific structure:


def function_name():
    # code inside the function

def tells Python you are defining a function
function_name is the name you choose (use lowercase and underscores)
The parentheses () are required
The colon : tells Python the function code is starting
All lines inside the function must be indented

Defining a Function


def say_hello():
    print("Hello!")

This defines the function but does not run it yet.

Indentation Matters

Python uses indentation to know which lines belong to a function.


def show_message():
    print("This line is inside the function")
print("This line is outside the function")

Only the indented line runs when the function is called.

Calling a Function

To run a function, write its name followed by parentheses.


say_hello()

The code inside the function runs from top to bottom.

Functions Can Have Multiple Lines


def start_program():
    print("Program starting...")
    print("Loading...")
    print("Ready!")
 
start_program()

Why Functions Are Useful

Organize code
Reduce repetition
Make programs easier to read
Control when code runs

Functions group instructions together so you can use them whenever you need.

Modules

A module is a file that contains extra code you can use in your program. Modules let you use features that are already written, instead of creating everything from scratch.

Python comes with many built-in modules.

Importing a Module

To use a module, you must import it first.


import random

This gives your program access to everything inside the random module.

Using a Module

After importing, you use the module name followed by a dot . and the function you want.


import random
 
number = random.randint(1, 10)
print(number)

random.randint(1, 10) gives a random number between 1 and 10

Why Modules Matter

Save time
Keep code clean
Use powerful tools easily
Share code between programs