Python 中的类和对象如何工作？

Question

Python 中的类和对象如何工作？

Accepted Answer

**类**是创建**对象**（实例）的蓝图，将数据（属性）和行为（方法）捆绑在一起。Python 的 OOP 很灵活，有一些独特的约定，特别是显式的 `self`。

## 定义一个类

```python
class Dog:
    species = "Canis familiaris"     # CLASS attribute (shared by all instances)

def __init__(self, name, age):   # the constructor — runs on creation
        self.name = name              # INSTANCE attributes (per object)
        self.age = age

def bark(self):                   # a method — `self` is the instance
        return f"{self.name} says woof"

d = Dog("Rex", 3)        # create an instance (calls __init__)
d.name                   # "Rex"
d.bark()                 # "Rex says woof"
```

## 显式 self

```python
def bark(self):          # `self` is ALWAYS the first parameter of a method
    return self.name      # access instance data through self
```

与许多具有隐式 `this` 的语言不同，Python 要求将 **`self`** 作为每个实例方法的显式第一个参数——它引用调用该方法的实例。你无需手动传递它；`d.bark()` 会自动将 `d` 作为 `self` 传递。

## 类属性 vs 实例属性

```python
Dog.species          # "Canis familiaris" — shared by all dogs
d.species            # also accessible from an instance
d.name               # "Rex" — unique to this instance
```

类属性在所有实例间共享；实例属性（通过 `self.x =` 设置）是逐对象的。

## 方法类型

```python
class Circle:
    def area(self): ...                    # instance method — needs self

@classmethod
    def from_diameter(cls, d):             # class method — receives the CLASS (cls)
        return cls(d / 2)                   # often used as alternative constructors

@staticmethod
    def pi(): return 3.14159               # static — no self/cls, just namespaced
```

## 封装约定

```python
self.public                # public
self._protected            # "internal use" by convention (not enforced)
self.__private             # name-mangled to _ClassName__private (stronger hiding)
```

Python 没有真正的私有成员——下划线前缀是约定，表示意图（