Defining functions

A function is a named, reusable machine: inputs go in (parameters) and a result comes out (return).

def area(width, height):
    return width * height

print(area(3, 4))   # 12

def only defines the machine. Nothing runs until you call it with parentheses. Parameters such as width receive the arguments supplied in the call. return ends the call and hands back its value. pass is a placeholder that does nothing; replace it with the function's real work.

Step through print(area(3, 4)): an area() box appears holding width 3 and height 4, shows the return value 12, then disappears.

See it run

def area(width, height):
    result = width * height
    return result

big = area(3, 4)
print(big)

Watch for a second box to appear: every function call gets its own.

Practice defining functions

Exercise 1

Define square(n) that returns n * n.
Define greet(name) that returns "Hi, NAME!" (e.g. greet("Bo") gives "Hi, Bo!").

Exercise 2

A picture framer needs perimeter(width, height) to return the distance around a rectangle and label(name, cm) to return text such as Oak: 18 cm. Zero dimensions are allowed. Do not print instead of returning.

Exercise 3

Split a cafe bill with two reusable functions. with_tip(amount, percent) returns amount plus that percentage tip. share(amount, people) returns each person's equal share. Compose them into each for a bill of 80, a 10% tip, and 4 people. Assume people is positive.

Exercise 4

Predict, then verify. Read this program before running it:

def double(x):
    result = x * 2
    return result

def add_then_double(a, b):
    total = a + b
    return double(total)

answer = add_then_double(3, 4)

While double is running, what is x in its frame, and how many boxes does the Visualizer show (count Program variables too)? Guess, then run it, press Step through your code, and press Next until a double() box appears. Set x_value and boxes.

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Practice

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Visualizer Watch your code run line by line

Run your code, then choose Play to follow its execution.

Memory · names → values

Variables appear here as Python creates them.

Output & plots