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Lesson 9 of 26 · about 15 min

Functions: your own instructions

def creates a new instruction with a name. Parameters are its inputs, and return sends a result back to whoever called it.

🧭 What you need first: Meet Bit the robot

The story

Think about a blender. You don’t need to know how the motor works. You put things in (banana, milk), press the button, and get something out (a smoothie). Different ingredients in, different smoothie out, same blender.

A function is a blender you build yourself. You write the recipe once, give it a name, and from then on you just “press the button” by calling its name. The inputs are called parameters, and what comes out is the return value.

You’ve already used functions someone else built: print(), input(), len(), move(). Now you’ll make your own.

Another way: a vending machine

Put in a code (input), get a snack (output). The machine does the same steps every time; only the input changes. You don’t care what happens inside, only what goes in and what comes out.

Another way: a recipe card titled “Pancakes”

Once a recipe card exists, you can just say “make pancakes” instead of listing every step. “Make pancakes for 4” changes one input. A function is a recipe card your program can refer to by name.

Play with it

Watch a function get called

Step through. Notice that def doesn’t run the body; it just creates the function. The body only runs when line 5 calls it, and a new box appears for that call.

Watch the code run, one step at a time

  1. 1def area(width, height):
  2. 2 result = width * height
  3. 3 return result
  4. 4
  5. 5kitchen = area(3, 4)
  6. 6hallway = area(1, 6)
  7. 7print(kitchen + hallway)

➜ yellow = the line about to run · ✓ = the line that just ran
Name it

The real words

def area(width, height):      # header: name and parameters
    """Return the area of a rectangle."""   # docstring
    return width * height      # body
  • def defines a function. Defining it doesn’t run it.
  • width and height are parameters: placeholder names for inputs.
  • area(3, 4) calls the function. 3 and 4 are the arguments, the actual values. Python matches them to the parameters in order.
  • return sends a value back to wherever the function was called and stops the function immediately.
  • No return? The function gives back the special value None (“nothing”).
  • The text in triple quotes is a docstring: a short description of what the function does. It’s for humans, and tools show it as help.

print vs return, the classic confusion:

print(x) return x
What it does shows text on screen hands a value back to the code
Can you use the result? no, it’s just displayed yes: store it, add it, compare it

Why functions? Write once, use many times (no copy-pasting). Give a name to an idea so code reads like English. Fix a bug in one place.

Build it

Write two functions

Write to_fahrenheit(celsius) that returns the temperature in Fahrenheit (multiply by 9, divide by 5, add 32). Then write is_even(n) that returns True if n is even, False if not.
Challenge

print or return?

def double(x): print(x * 2) …then: y = double(5) and print(y + 1). What happens?

Write longest(a, b) that returns whichever string is longer (if they're the same length, return a).
Remember it

In def greet(name): … greet("Sam"), which is the parameter and which is the argument?

Got it?

Finish the lesson to color it in on your map and collect its concept cards.

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