Python高级：了解Python ABC（抽象基类）及应用场景

太过爱你忘了你带给我的痛 2022-05-25 08:30 268阅读 0赞

ABC，Abstract Base Class（抽象基类），主要定义了基本类和最基本的抽象方法，可以为子类定义共有的API，不需要具体实现。相当于是Java中的接口或者是抽象类。

抽象基类可以不实现具体的方法（当然也可以实现，只不过子类如果想调用抽象基类中定义的方法需要使用super()）而是将其留给派生类实现。

抽象基类提供了逻辑和实现解耦的能力，即在不同的模块中通过抽象基类来调用，可以用最精简的方式展示出代码之间的逻辑关系，让模块之间的依赖清晰简单。同时，一个抽象类可以有多个实现，让系统的运转更加灵活。而针对抽象类的编程，让每个人可以关注当前抽象类，只关注其方法和描述，而不需要考虑过多的其他逻辑，这对协同开发有很大意义。极简版的抽象类实现，也让代码可读性更高。

抽象基类的使用：

1：直接继承

直接继承抽象基类的子类就没有这么灵活，抽象基类中可以声明”抽象方法“和“抽象属性”，只有完全覆写（实现）了抽象基类中的“抽象”内容后，才能被实例化，而虚拟子类则不受此影响。

2：虚拟子类

将其他的类”注册“到抽象基类下当虚拟子类（调用register方法），虚拟子类的好处是你实现的第三方子类不需要直接继承自基类，可以实现抽象基类中的部分API接口，也可以根本不实现，但是issubclass(), issubinstance()进行判断时仍然返回真值。

Python 对于ABC的支持模块是abc模块，定义了一个特殊的metaclass：ABCMeta 还有一些装饰器：@abstractmethod 和 @abstarctproperty 。abc.ABCMeta 用于在Python程序中创建抽象基类。而抽象基类如果想要声明“抽象方法”，可以使用 @abstractmethod ，如果想声明“抽象属性”，可以使用 @abstractproperty 。

为了解决Python2&3的兼容问题，需要引入six模块，该模块中有一个针对类的装饰器 @six.add\_metaclass(MetaClass) 可以为两个版本的Python类方便地添加metaclass

实例：

<table style="border-spacing:0px;width:852px;color:rgb(51,51,51);font-size:14px;background-color:rgb(255,255,255);table-layout:fixed;"> 
 <tbody> 
  <tr> 
   <td style="padding:10px;margin:0px;border-color:rgb(219,219,219);min-width:100%;"> 
    <div style="padding:0px;margin:0px;">
      import abc 
    </div> 
    <div style="padding:0px;margin:0px;">
      import six 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      @six.add_metaclass(abc.ABCMeta) 
    </div> 
    <div style="padding:0px;margin:0px;">
      class BaseClass(object): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;@abc.abstractmethod 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;def func_a(self, data): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;""" 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;an abstract method need to be implemented 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;""" 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;@abc.abstractmethod 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;def func_b(self, data): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;""" 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;another abstract method need to be implemented 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;""" 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      class SubclassImpl(BaseClass): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;def func_a(self, data): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;print("Overriding func_a, " + str(data)) 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;@staticmethod 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;def func_d(self, data): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;print(type(self) + str(data)) 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      class RegisteredImpl(object): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;@staticmethod 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;def func_c(data): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;print("Method in third-party class, " + str(data)) 
    </div> 
    <div style="padding:0px;margin:0px;">
      BaseClass.register(RegisteredImpl) 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      if __name__ == '__main__': 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;for subclass in BaseClass.__subclasses__(): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;print("subclass of BaseClass: " + subclass.__name__) 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("subclass do not contains RegisteredImpl") 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("-----------------------------------------------") 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("RegisteredImpl is subclass: " + str(issubclass(RegisteredImpl, BaseClass))) 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("RegisteredImpl object&nbsp;&nbsp;is instance: " + str(isinstance(RegisteredImpl(), BaseClass))) 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("SubclassImpl is subclass: " + str(issubclass(SubclassImpl, BaseClass))) 
    </div> 
    <div style="padding:0px;margin:0px;"> 
     <br> 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("-----------------------------------------------") 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;obj1 = RegisteredImpl() 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;obj1.func_c("RegisteredImpl new object OK!") 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;print("-----------------------------------------------") 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;obj2 = SubclassImpl()&nbsp;&nbsp;#由于没有实例化所有的方法，所以这里会报错 Can't instantiate abstract class SubclassImpl with abstract methods func_b 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;obj2.func_a("It's right!") 
    </div></td> 
  </tr> 
  <tr> 
   <td style="padding:10px;margin:0px;border-color:rgb(219,219,219);min-width:100%;"> 
    <div style="padding:0px;margin:0px;">
      结果如下： 
    </div> 
    <div style="padding:0px;margin:0px;">
      subclass of BaseClass: SubclassImpl 
    </div> 
    <div style="padding:0px;margin:0px;">
      subclass do not contains RegisteredImpl 
    </div> 
    <div style="padding:0px;margin:0px;">
      ----------------------------------------------- 
    </div> 
    <div style="padding:0px;margin:0px;">
      RegisteredImpl is subclass: True 
    </div> 
    <div style="padding:0px;margin:0px;">
      RegisteredImpl object&nbsp;&nbsp;is instance: True 
    </div> 
    <div style="padding:0px;margin:0px;">
      SubclassImpl is subclass: True 
    </div> 
    <div style="padding:0px;margin:0px;">
      ----------------------------------------------- 
    </div> 
    <div style="padding:0px;margin:0px;">
      Method in third-party class, RegisteredImpl new object OK! 
    </div> 
    <div style="padding:0px;margin:0px;">
      ----------------------------------------------- 
    </div> 
    <div style="padding:0px;margin:0px;">
      Traceback (most recent call last): 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;File "/Users/wangqi/Git/Python/scrapy_crawler_learn/test/ABCTest.py", line 51, in &lt;module&gt; 
    </div> 
    <div style="padding:0px;margin:0px;">
      &nbsp;&nbsp;&nbsp;&nbsp;obj2 = SubclassImpl()&nbsp;&nbsp;#由于没有实例化所有的方法，所以这里会报错 Can't instantiate abstract class SubclassImpl with abstract methods func_b 
    </div> 
    <div style="padding:0px;margin:0px;">
      TypeError: Can't instantiate abstract class SubclassImpl with abstract methods func_b 
    </div></td> 
  </tr> 
 </tbody> 
</table>

1. \#我们去检查某个类是否有某种方法  
class Company(object):  
    def \_\_init\_\_(self, employee\_list):  
        self.employee = employee\_list  
  
  
    def \_\_len\_\_(self):  
        return len(self.employee)  
  
  
  
  
com = Company(\["bobby1","bobby2"\])  
print(hasattr(com, "\_\_len\_\_"))  
  
  
  
  
class A:  
    pass  
  
  
class B:  
    pass  
  
  
2.  \#我们在某些情况之下希望判定某个对象的类型  
from collections.abc import Sized  
isinstance(com, Sized)  
  
  
b = B()  
print(isinstance(b, A))  
\# print(len(com))  
  
3.   
\#我们需要强制某个子类必须实现某些方法  
\#实现了一个web框架，集成cache(redis, cache, memorychache)  
\#需要设计一个抽象基类， 指定子类必须实现某些方法  
  
  
\#如何去模拟一个抽象基类  
  
  
import abc  
from collections.abc import \*  
  
  
  
  
class CacheBase(metaclass=abc.ABCMeta):  
    @abc.abstractmethod  
    def get(self, key):  
        pass  
  
  
    @abc.abstractmethod  
    def set(self, key, value):  
        pass  
\# class CacheBase():  
\#     def get(self, key):  
\#         raise NotImplementedError  
\#     def set(self, key, value):  
\#         raise NotImplementedError  
\#  
class RedisCache(CacheBase):  
    def set(self, key, value):  
        pass  
  
  
\# redis\_cache = RedisCache()  
\# redis\_cache.set("key", "value")