Lesson 28 · Designing code

Objects and classes

A class is a blueprint and objects are the things you build from it, each with its own data. Step through a small BankAccount class in Python: build two accounts with __init__, call methods on them, see why the balance is changed only through methods, then make a Savings subclass that reuses and overrides code, and a Bank that has accounts instead of being one.

12 min10 steps5 questionsIntermediate

Fig 28 · Interactive

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5 quick questions

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01After x = BankAccount("Xia") and y = BankAccount("Yan") , then x.deposit(20) , what is y._balance?

x and y are two separate objects built from the same class. Each has its own _balance, so depositing into x leaves y at 0.

02Inside def deposit(self, amount): , what is self when you call x.deposit(20) ?

x.deposit(20) is the same as BankAccount.deposit(x, 20): Python passes the object the method was called on as the first argument, which is named self.

03Savings inherits from BankAccount and defines only withdraw. What runs for s.deposit(30) ?

When a method is not found on the subclass, Python looks in its parent class. A subclass reuses every method it does not override.

04for acc in (a, s): acc.withdraw(5) takes 5 from a but 6 from s. What is this called?

Polymorphism: the same method name works on objects of different classes, and each object runs its own class's version. Savings' withdraw adds a fee of 1.

05A Car needs an Engine. Which design fits best?

A car is not an engine, it has one. That is a has-a relationship, so composition (an attribute holding an Engine object) fits; keep inheritance for a true is-a.

Key terms

Glossary

Class: A blueprint, written with class Name:, that describes what its objects hold and what they can do.
Object (instance): One thing built from a class, with its own attributes (data), such as a.owner and a._balance.
Method and self: A function defined in a class. Calling a.deposit(50) runs it with self set to the object a.
__init__: The method Python runs on every new object right after making it, to give it its starting attributes.
Encapsulation: Keeping an object's data behind its methods, which check the rules. In Python a leading underscore, as in _balance, marks data as internal.
Inheritance and overriding: A subclass, class Savings(BankAccount):, reuses its parent's methods; defining a method with the same name overrides the parent's version.
Polymorphism: The same method call working on objects of different classes, each running its own version.
Composition: Building an object out of other objects held in its attributes (has-a), such as a Bank holding a list of accounts.

Fine print

Notes and sources

a.deposit(50) is exactly equivalent to BankAccount.deposit(a, 50). That is why every method's first parameter, named self by convention, receives the object the method was called on.
Python has no truly private attributes. A leading underscore, as in _balance, is a convention that marks a non-public part: code outside can still reach a._balance, but should not.
When an object and its class lack an attribute, Python searches the parent classes in a fixed order, the method resolution order, shown by Savings.__mro__.
"Favor object composition over class inheritance" is a principle from Gamma, Helm, Johnson and Vlissides, Design Patterns (1994). Think Python names the two relationships IS-A (inheritance) and HAS-A (composition).
The cards indent by two spaces so each line fits. Python only needs the indentation to be consistent; the style guide PEP 8 recommends four spaces.
The fee of 1 on Savings withdrawals is invented for the example.

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