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factory method python codes, Implementing an Interface in Python,…

- - - - regular class
        
        构造方法、普通方法、static方法、常量和变量
        
        static vs dynamic
        
        类（class）通过 static 关键字定义静态方法。不能在类的实例上调用静态方法，而应该通过类本身调用。这些通常是实用程序方法，例如创建或克隆对象的功能。
        
        抽象类是指在普通类的结构里面增加抽象方法的组成部分。
        
        在所有的普通方法上面都会有一个“{}”，这个表示方法体，有方法体的方法一定可以被对象直接使用。而抽象方法，是指没有方法体的方法，同时抽象方法还必须使用关键字abstract做修饰。
        
        拥有抽象方法的类就是抽象类，抽象类要使用abstract关键字声明。
        
        （1）抽象方法必须为public或者protected（因为如果为private，则不能被子类继承，子类便无法实现该方法），缺省情况下默认为public；
        
        （2）抽象类不能直接实例化，需要依靠子类采用向上转型的方式处理；
        
        （3）抽象类必须有子类，使用extends继承，一个子类只能继承一个抽象类；
        
        （4）子类（如果不是抽象类）则必须覆写抽象类之中的全部抽象方法（如果子类没有实现父类的抽象方法，则必须将子类也定义为为abstract类。）；
        
        所有的子类如果要想正常的完成操作，必须按照指定的方法进行覆写才可以，而这个时候抽象类所起的功能就是一个类定义模板的功能
        
        Java抽象类详解
        
        A是抽象的，无法直接进行实例化操作。为什么不能直接实例化呢？当一个类实例化之后，就意味着这个对象可以调用类中的属性或者方法，但在抽象类里存在抽象方法，而抽象方法没有方法体，没有方法体就无法进行调用。既然无法进行方法调用的话，又怎么去产生实例化对象呢。
- - - - To use your interface, you must create a concrete class. A concrete class is a subclass of the interface that provides an implementation of the interface’s methods.
        
        Such informal interfaces are fine for small projects where only a few developers are working on the source code. However, as projects get larger and teams grow, this could lead to developers spending countless hours looking for hard-to-find logic errors in the codebase!
  - - - you create a class called UpdatedInformalParserInterface that builds from the ParserMeta metaclass:
        
        you can create your concrete implementations.
        
        have a look at the MRO:
        
        UpdatedInformalParserInterface is a superclass of PdfParserNew, but it doesn’t appear in the MRO. This unusual behavior is caused by the fact that UpdatedInformalParserInterface is a virtual base class of PdfParserNew.
        
        virtual base classes use the .subclasscheck() dunder method to implicitly check if a class is a virtual subclass of the superclass. Additionally, virtual base classes don’t appear in the subclass MRO.
        
        you have a metaclass that’s used to create UpdatedInformalParserInterface. By using a metaclass, you don’t need to explicitly define the subclasses. Instead, the subclass must define the required methods. If it doesn’t, then issubclass(EmlParserNew, UpdatedInformalParserInterface) will return False.
- - - - Once you’ve imported the abc module, you can directly register a virtual subclass by using the .register() metamethod.
        
        Once you’ve registered Double, you can use it as class decorator to set the decorated class as a virtual subclass:
        
        The decorator register method helps you to create a hierarchy of custom virtual class inheritance.
    - - Rather than create your own metaclass, you’ll use abc.ABCMeta as the metaclass.
        
        Python’s builtin ABCMeta from the abc module.
        
        ou’ll overwrite .subclasshook() in place of .instancecheck() and .subclasscheck(), as it creates a more reliable implementation of these dunder methods.
    - - Please use caution with virtual subclass registration!
    - - An abstract method is a method that’s declared by the Python interface, but it may not have a useful implementation. The abstract method must be overridden by the concrete class that implements the interface in question.
        
        To create abstract methods in Python, you add the abc.abstractmethod decorator to the interface’s methods.
        
        created a formal interface that will raise errors when the abstract methods aren’t overridden.