Python abc抽象基类的虚拟子类机制

abc.ABCMeta元类通过register注册虚拟子类。__subclasshook__自动判断子类关系。@abstractmethod标记抽象方法。@abstractproperty/abstractclassmethod/abstractstaticmethod在较新Python使用组合装饰器。__isabstractmethod__控制抽象性。register不检查方法实现只声明关系。__subclasshook__基于方法存在性自动判断。ABC与Protocol异同。鸭子类型利用__subclasshook__兼容。

from abc import ABC, abstractmethod

class Stream(ABC):
@abstractmethod
def read(self): pass

@abstractmethod
def write(self, data): pass

@classmethod
def __subclasshook__(cls, C):
if cls is Stream:
if (any('read' in B.__dict__ for B in C.__mro__) and
any('write' in B.__dict__ for B in C.__mro__)):
return True
return NotImplemented

class FileHandler:
def read(self): return "data"
def write(self, data): print(f"Write: {data}")

print(issubclass(FileHandler, Stream)) # True
print(isinstance(FileHandler(), Stream)) # True