Objects and classes

One home for each book

• A library app tracks thousands of books. If titles and copy counts sit in loose variables, one slip updates the wrong book and nobody can tell where.
• Object-oriented programming 面向对象程序设计 gives each book one home: an object that owns its own data and actions.

Classes and instances

• A class 类 is a blueprint. An object 对象 is one thing built from it, also called an instance 实例 of the class.
• Book is the class; the copy of Signals on the shelf is one instance, and Maps is another.

The constructor and self

• __init__ runs when an object is created and sets up its instance attributes 实例属性. Every method takes self, which is the instance itself.

class Book:
    next_id = 1    # shared
    def __init__(self, title, n):
        self.id = Book.next_id
        Book.next_id += 1
        self.title = title
        self.copies = n
    def take(self):
        self.copies -= 1
        return self.copies

Class attributes are shared

• Class attributes 类属性 live on the class itself, and every instance reads the same one. Book.next_id belongs there so every caller agrees on the next number.
• Instance attributes such as title and copies are separate for each object.

Names are references

• Assignment never copies an object. A variable holds a reference 引用 to it, so c = a makes two aliases 别名 for one object.
• A change made through either name is visible through both.

Equality and identity

• Equality 相等 (==) compares values, once a class defines how. Identity 同一 (is) asks whether two names point to the very same object.
• If a class does not define ==, Python falls back to identity, so two separate books with the same title are not equal.

Where Python looks for an attribute

• obj.x is looked up from the inside out: the instance first, then the class, then the base classes.
• So a.next_id = 9 creates an instance attribute that hides the class value for a only. Book.next_id is untouched.

Worked example: trace the state

Three common slips

Recap