Unit 12: Your Own Types
Building things with classes.
Unit 12 of 31 in Python for kids. Its 5 lessons are Blueprints, Things That Do Things, Printing Nicely, Lots of Objects and Class Master — 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.
📘 Blueprints
A shape for your own kind of thing
A class is a blueprint. It does nothing on its own — you use it to build objects, and each one holds its own information.
Python
class Dog:
def __init__(self, name):
self.name = name
rex = Dog("Rex")
print(rex.name)It prints
Rex
What __init__ is for
__init__ runs the moment you build one. Its job is to put the starting information onto the new object. self is the object being built.
Python
class Dog:
def __init__(self, name, age):
self.name = name
self.age = age
rex = Dog("Rex", 4)
print(rex.name, rex.age)It prints
Rex 4
Each object keeps its own
Build two and they do not share anything. Changing one leaves the other exactly as it was.
Python
class Dog:
def __init__(self, name):
self.name = name
a = Dog("Rex")
b = Dog("Bo")
print(a.name)
print(b.name)It prints
Rex Bo
Try it yourself
What is self?
- The name of the class
- The particular object being worked on right now
- A Python keyword you must never change
- The file you are in
What does this print?
Python
class Cat:
def __init__(self, lives):
self.lives = lives
c = Cat(9)
c.lives = c.lives - 1
print(c.lives)Answer them in the app
⚡ Things That Do Things
A function that lives inside a class
A function inside a class is called a method. It always takes self first, so it can reach the object it belongs to.
Python
class Dog:
def __init__(self, name):
self.name = name
def speak(self):
print(self.name + " says woof")
Dog("Rex").speak()It prints
Rex says woof
Methods can change the object
A method can update what the object stores, so the change lasts after the method finishes.
Python
class Counter:
def __init__(self):
self.count = 0
def bump(self):
self.count = self.count + 1
c = Counter()
c.bump()
c.bump()
print(c.count)It prints
2
Try it yourself
Why does every method start with self?
- Python just likes it
- So the method can see and change the object it was called on
- To make it run faster
- It is optional
What does this print?
Python
class Box:
def __init__(self):
self.items = []
def add(self, thing):
self.items.append(thing)
b = Box()
b.add("hat")
b.add("map")
print(len(b.items))Answer them in the app
🖨️ Printing Nicely
Printing an object is ugly by default
Print an object and Python shows something like <__main__.Dog object at 0x7f8b1c>, which helps nobody. __str__ lets you say what it should look like instead.
Python
class Dog:
def __init__(self, name):
self.name = name
def __str__(self):
return "a dog called " + self.name
print(Dog("Rex"))It prints
a dog called Rex
It works inside f-strings too
Anywhere Python needs your object as text, it calls __str__.
Python
class Pet:
def __init__(self, name):
self.name = name
def __str__(self):
return self.name.upper()
p = Pet("tig")
print(f"My pet is {p}")It prints
My pet is TIG
Try it yourself
What must __str__ do?
- Print the text
- Return the text
- Both print and return it
- Nothing, Python fills it in
Answer it in the app
👥 Lots of Objects
A list of your own things
Objects go in lists like anything else, and then a loop can work through them.
Python
class Pet:
def __init__(self, name, legs):
self.name = name
self.legs = legs
pets = [Pet("cat", 4), Pet("bird", 2)]
for pet in pets:
print(pet.name, pet.legs)It prints
cat 4 bird 2
Adding them up
Once they are in a list you can total, count or filter them just like numbers.
Python
class Pet:
def __init__(self, legs):
self.legs = legs
pets = [Pet(4), Pet(2), Pet(4)]
total = 0
for pet in pets:
total = total + pet.legs
print(total)It prints
10
Sorting your own objects
sorted needs to know *what* to sort by. key takes a small function saying which part to look at.
Python
class Kid:
def __init__(self, name, age):
self.name = name
self.age = age
kids = [Kid("Ada", 11), Kid("Sam", 7)]
for kid in sorted(kids, key=lambda k: k.age):
print(kid.name)It prints
Sam Ada
Try it yourself
What does this print?
Python
class Kid:
def __init__(self, age):
self.age = age
kids = [Kid(7), Kid(11), Kid(9)]
older = []
for kid in kids:
if kid.age > 8:
older.append(kid.age)
print(older)Answer it in the app
🏆 Class Master
Try it yourself
What does this print?
Python
class Tally:
def __init__(self):
self.n = 0
def up(self):
self.n = self.n + 2
t = Tally()
t.up()
t.up()
t.up()
print(t.n)Answer it in the app