Python类变量与实例变量

"""Python类变量与实例变量——属性查找链与可变对象陷阱
------------------------------------------------------------------------------
属性访问遵循 实例 -> 类 -> 父类 的查找链。类变量在实例间共享，
实例变量每个对象独有一份。理解边界是避免诡异 bug 的关键。
"""

from typing import List, Optional


# ========== 1. 属性查找链：实例变量遮蔽类变量 ==========

class Employee:
company: str = "Acme Corp" # 类变量共享
def __init__(self, name: str) -> None:
self.name = name # 实例变量独有

e1, e2 = Employee("Alice"), Employee("Bob")
print(e1.company) # Acme Corp（查找到类变量）

e1.company = "Startup" # 创建实例变量遮蔽类变量
print(e1.company) # Startup（实例变量）
print(e2.company) # Acme Corp（类变量不变）
del e1.company # 删除后恢复类变量可见
print(e1.company) # Acme Corp


# ========== 2. 可变类变量陷阱 ==========

class Group:
"""可变类变量（列表/字典）会在所有实例间共享！"""
students: List[str] = [] # 错误的共享设计
def __init__(self, name: str) -> None:
self.name = name
def add(self, name: str) -> None:
self.students.append(name)

g1, g2 = Group("一班"), Group("二班")
g1.add("小明"); g2.add("小红")
print(g1.students) # ['小明', '小红'] —— 互相干扰！
print(g2.students) # ['小明', '小红'] —— 同一引用

class SafeGroup:
"""正确：实例变量在 __init__ 中初始化"""
max_capacity = 50 # 不可变常量放类变量没问题
def __init__(self, name: str) -> None:
self.name = name
self.students: List[str] = [] # 每个实例独立列表


# ========== 3. @classmethod 访问类变量 ==========

class Config:
_cfg: dict = {"debug": False}
_count: int = 0
def __init__(self) -> None:
Config._count += 1
@classmethod
def get(cls, key: str) -> Optional[str]:
return cls._cfg.get(key)
@classmethod
def set(cls, key: str, val: str) -> None:
cls._cfg[key] = val


# ========== 4. __dict__ 检查 ==========

class Parent:
p_var = "parent"
shared: List[str] = []
class Child(Parent):
c_var = "child"

obj = Child(); obj.instance = "owned"
print(obj.__dict__) # {'instance': 'owned'}
print("c_var" in Child.__dict__) # True
print("p_var" in Child.__dict__) # False（继承，不在自身 __dict__）


# ========== 5. 继承与可变类变量共享 ==========

class Base:
label = "base"
items: List[str] = []
class A(Base): label = "A"
class B(Base): label = "B"

print(A.label, B.label) # A B（不可变独立）
A.items.append("来自A")
print(B.items) # ['来自A'] —— 基类列表共享！