Functions

Why this matters

When the same steps appear again and again, you wrap them in a function and give them a name. Then you can run those steps any time just by calling the name, with different inputs each time. Functions keep programs short, clear, and reusable.

The idea

You define a function with def, list its arguments in parentheses, and send a result back with return:

def function_name(argument1, argument2):
    # process
    return result

• Arguments are the inputs the function receives.
• return hands a return value back to the caller; the function stops there.
• Calling the function means writing its name with values in parentheses, e.g. area(10, 5).

Python already gives you built-in functions like print(); the ones you write yourself are user-defined functions.

Picture it

flowchart TD
  A([Call area with 10, 5]) --> B[base = 10, height = 5]
  B --> C[S = base * height / 2]
  C --> D[return S]
  D --> E([caller receives 25.0])

Worked example

def area(base, height):
    s = base * height / 2
    return s

print(area(10, 5))   # 25.0
print(area(6, 7))    # 21.0

The arguments 10 and 5 flow into base and height. The function computes s and returns it, so print shows 25.0. Call it again with new values and you get a new result.

Your turn

Write functions that take arguments and return a computed value, and predict what a function call prints.

Recap

• def name(args): defines a function; return sends back the return value.
• Arguments are inputs; calling runs the function with real values.
• print() is a built-in function; the ones you write are user-defined.