Functions — PythonLessonLab

THE IDEA

Understand the concept

A function groups instructions under a name. Parameters receive values when the function is called. The body runs only when you call the function.

return sends a value back to the caller. print only displays a value; it does not make that value the function's result. Names created inside a function are normally local to it.

Define with def
Pass arguments into parameters
return a reusable result

WORKED EXAMPLES

See it step by step

Read the code, predict the output, then compare it with the result.

01. Return a greeting

PYTHON

def greet(name):
    return f"Hi, {name}!"

print(greet("Maya"))

EXPECTED OUTPUT

Hi, Maya!

The argument Maya becomes the local name parameter. return sends the finished string back to print().

02. Reuse a calculation

PYTHON

def triple(number):
    return 3 * number

print(triple(4))
print(triple(7))

EXPECTED OUTPUT

12
21

One function can be called with different arguments without repeating the calculation.

HANDS-ON

Try it in Python

Edit the example and run it. Python starts in your browser the first time you click Run.

Python console

Ready to run

def square(value):
    return value * value

answer = square(4)
print(answer)
print(square(7))

Need input()? Add one value per line

Input lines

Ctrl / ⌘ + Enter to run

OUTPUT

Your output appears here.

Your turnCreate a function named double that returns twice its argument. Call it with 9 and print the result.

Need a hint?

Define def double(value): and return value * 2 from inside it. After the definition, call double(9) inside print().

Complete the task and select Check task to verify your code.

KNOWLEDGE CHECK

Quick quiz

Three questions. You can change your answers and try again.

DEBUG BETTER

Typical mistakes

Everyone meets these errors. See what causes them and how to fix them.

Printing instead of returning

COMMON MISTAKE

def double(value):
    print(value * 2)
result = double(4)
print(result)

FIX

def double(value):
    return value * 2
result = double(4)
print(result)

What happens: The function prints 8, but result is None.
Use return when callers need to store or reuse the calculated value.

Missing required argument