Functions

Package reusable behavior behind clear inputs and outputs.

Why this matters

Functions help you avoid duplication and give a meaningful name to a unit of work.

Start with the idea

A function can receive parameters, perform work, and return a result. Good functions are small enough to understand and named after what they accomplish.

IdeaUnderstand the purpose

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CodeRun the smallest example

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PracticeChange it yourself

SubjectVision learning pattern: understand the idea before memorizing syntax.

Small working example

Run this example first. Do not change several things at once; confirm the basic behavior, then experiment.

def calculate_total(price, quantity):
    return price * quantity

print(calculate_total(8, 3))

Expected output

What to notice

The example is intentionally small. Focus on the chapter’s main idea before adding extra syntax or framework code.

Common beginner mistake

Mistake

Printing a value inside a function when callers actually need the value returned.

Better approach

Use return when the result should be reusable by other code; print only when displaying output is the function's job.

Quick recap

Functions name reusable behavior.
Parameters are inputs.
return sends a result back to the caller.

Try it yourself

These are deliberately small. If you can complete them without copying the example, you are ready to continue.

Write a function that calculates tax.
Call the same function with three different inputs.