Unit 8: Smarter Functions

Defaults, scope and recursion.

Unit 8 of 31 in Python for kids. Its 5 lessons are Ready-Made Answers, More Than One Answer, Where Names Live, Functions That Call Themselves and Function Workshop — below is everything each one explains, and a question or two from it to try.

Every sample on this page was run through real Python before it shipped, and prints exactly what it says it prints.

This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.

🎁 Ready-Made Answers

An argument that fills itself in

Give a parameter a value in the def line and it becomes optional. Leave it out when you call, and the ready-made value is used.

Python

def greet(name, greeting="Hello"):
    print(greeting + " " + name)

greet("Ada")
greet("Sam", "Howdy")

It prints

Hello Ada
Howdy Sam

Naming the argument

You can also say which parameter you mean by name. Handy when a function has several optional ones and you only want to change the last.

Python

def draw(shape="star", size=1):
    print(shape * size)

draw(size=3)

It prints

starstarstar

Try it yourself

What does this print?

Python

def shout(word, times=2):
    print(word * times)

shout("hi")
shout("hi", 3)

Where must parameters with a default go?

First, before the ordinary ones
Last, after the ordinary ones
Anywhere
On their own line

Answer them in the app

🎒 More Than One Answer

Returning two things at once

Separate values with a comma after return and the function hands back all of them. Catch them with matching names on the left.

Python

def best_and_worst(scores):
    return max(scores), min(scores)

high, low = best_and_worst([4, 9, 2])
print(high)
print(low)

It prints

9
2

That bundle is a tuple

A tuple is a little list that can never be changed. Round brackets instead of square ones — and Python makes one for you whenever you write values with commas.

Python

point = (3, 7)
print(point)
print(point[0])

It prints

(3, 7)
3

Try it yourself

What is the difference between a tuple and a list?

A tuple holds only numbers
A tuple cannot be changed once it is made
A tuple is faster to print
There is no difference

What does this print?

Python

def split_name(full):
    return full[0], full[-1]

first, last = split_name("Ada")
print(first + last)

Answer them in the app

🏠 Where Names Live

A function has its own little world

A name created inside a function only exists inside it. When the function ends, the name is gone — which is why two functions can both use n without ever getting confused.

Python

def add():
    total = 5
    print(total)

add()
print("done")

It prints

5
done

Looking outwards is fine

A function can *read* a name from outside. It just cannot change one by ordinary assignment — that would quietly make a new local name instead.

Python

score = 10

def show():
    print(score)

show()

It prints

Better than global: return it

Python has a global keyword, but reaching out and changing outside names makes programs hard to follow. Returning the new value is nearly always tidier.

Python

score = 10

def bump(current):
    return current + 1

score = bump(score)
print(score)

It prints

Try it yourself

What happens if you print(total) *after* that function, outside it?

It prints 5
It prints 0
Python complains that total is not defined
It prints nothing

Careful — what does this print?

Python

score = 10

def bump():
    score = 99

bump()
print(score)

Answer them in the app

🪆 Functions That Call Themselves

Russian dolls

A function is allowed to call itself. That is called recursion, and it needs two parts: a way to stop, and a step that gets closer to stopping.

Python

def countdown(n):
    if n == 0:
        print("Liftoff!")
        return
    print(n)
    countdown(n - 1)

countdown(3)

It prints

3
2
1
Liftoff!

Building an answer on the way back

Each call waits for the one inside it to finish, then uses that answer. factorial(4) is 4 times whatever factorial(3) works out to be.

Python

def factorial(n):
    if n == 1:
        return 1
    return n * factorial(n - 1)

print(factorial(4))

It prints

Try it yourself

What is the stopping part called?

The end case
The base case
The stop case
The final case

What does this print?

Python

def total(n):
    if n == 0:
        return 0
    return n + total(n - 1)

print(total(4))

Answer them in the app

🏆 Function Workshop

Try it yourself

What does this print?

Python

def tag(word, mark="!"):
    return word + mark

print(tag("hi"))
print(tag("hi", "?"))

Answer it in the app