Classes and Inheritance: Quick Revision

A brief note on Classes and Inheritance. Read it before a test. Try each question at the end before you open its answer.

Objects and classes

• Every value in Python is an object. It has attributes (its data) and methods (its actions), reached with a dot.
• A class is a kind of thing. An object is one example of it. type(x) gives the class of x.
• A method is called with brackets, word.upper(). An attribute is read without them, z.real.

Writing a class

• class Name: with methods inside. __init__(self, ...) sets up each new object, and Name(arguments) makes one.
• self is the object that the method was called on. It is always the first parameter.
• self.level = 0 makes data that belongs to this one object. Each object has its own.
• The state of an object is the values of its attributes. A command changes the state; a question returns an answer. Keep the two jobs apart.
• The rules belong inside the class. Check a rule before you change the state, so a refused action changes nothing.
• A class attribute, written in the class body, is shared by all the objects. Writing to it through an object makes a new object attribute instead.
• b = a gives the same object a second name. It is not a copy.
• Never use a list as a default value. Write items=None, and make a new list inside __init__.

Inheritance

• class Child(Parent): builds one class on another. The child gets everything and adds its own.
• A child with its own __init__ calls super().__init__(...), so the parent sets up its part.
• A child overrides a method by writing one with the same name. super().method() runs the parent's version, to extend it.
• Polymorphism: the same call gives the right answer for each kind of object, and the code does not ask which kind it is.
• isinstance(x, Parent) is true for the parent and for every class built on it.
• Use inheritance only when "is a" is true. When one object keeps another in an attribute, that is composition, "has a". Prefer it.

Special methods

Python calls these for you. They have two underscores on each side.

• __str__ and __repr__ return text. They do not print it.

A worked example

A water tank keeps its level in litres, and refuses an action that breaks its rules.

Follow the state after each action, including the refused one.

1. The check comes first. In the refused fill, raise runs before self.level += litres, so the level stays at 320 litres.
2. Each object has its own state. A second Tank(800) would start at 0 litres, whatever t holds.

The program shows Refused: the tank would overflow and then 450 litres.

Practice with new numbers

A program to try

A cinema has three kinds of ticket. Two child classes override price, and __lt__ lets sorted put the tickets in order of price.

Mistakes to avoid

• No self in the def line. def price(): and then t.price() stops with TypeError: T.price() takes 0 positional arguments but 1 was given.
• No self. in __init__. capacity = capacity makes a local name that vanishes. Later t.capacity stops with AttributeError: 'Tank' object has no attribute 'capacity'.
• Forgetting super().__init__(...) in a child with its own __init__. The parent's attributes are never made, and reading one gives an AttributeError.
• A __str__ that prints. print(x) then stops with TypeError: __str__ returned non-string (type NoneType). Return the text.
• A misspelt override. A child method called prise adds a new method. The parent's price still runs, and nobody notices.
• Changing the state, then checking. The object is already broken when the error is raised.

Check yourself

1. A class Lamp sets self.on = False in __init__, and its method switch sets self.on = not self.on. A program runs a = Lamp(), b = a and b.switch(). What is a.on now?

2. A school program needs teachers and a timetable. Which is the better design: class Teacher(Timetable), or a Timetable that keeps a list of Teacher objects? Why?

3. A class Parcel stores self.kg and has __lt__ that compares self.kg < other.kg. For parcels of 3 kg, 7 kg and 5 kg, what do max(parcels) and sorted(parcels) give?

4. A class Bird has move(), which returns "flies". Its child Penguin defines def mvoe(self): return "swims". What does Penguin().move() give?

5. Why should the __add__ method of a class Distance return a new Distance, and not change self?