Python之抽象基类

抽象基类的核心定义在abc模块中，模块中包括了创建抽象基类需要的修饰符和元类型
abc.ABCMeta

　　用来生成抽象基础类的元类。由它生成的类可以被直接继承。
abc.ABC

　　辅助类，让你可以不用关心元类概念，直接继承它，就有了ABCMeta元类。使用时注意元类冲突
@abc.abstractmethod

　　定义抽象方法，除了这个装饰器，其余装饰器都被deprecated了

1. 抽象类示例　

from abc import ABCMeta,abstractmethod

class Animal(metaclass = ABCMeta):  
    test = "hello world"
    
    def __init__(self):
        self.food = None
        
    @abstractmethod
    def eat(self):
        pass

    @abstractmethod
    def run(self):
        pass

    @classmethod
    def __subclasshook__(cls, subclass):   #重写__subclasshook__方法，判断是否为子类
        print ("come in")
        if cls is Animal:
            if any("eat" in b.__dict__ for b in subclass.__mro__):
                return True
        return NotImplemented   

　　以上为Animal的抽象基类，注意重写了__subclasscheck__(cls, subclass)方法来改变issubclass或者isinstance的行为，__subclasscheck__(cls, subclass)必须为@classmethod

2. 具体化抽象类的两种方式

　　具体化抽象类可以有两种方式，一种通过注册（register），另外一种通过继承。

register方法　

class Monkey:
    def __init__(self):
　　　　　#不会出现在类的__mro__，所以不会通过super()方法调用基类方法
        super().__init__()  
        self.food= "banana"
        
    def eat(self):
        print ("{0} eat {1}".format(self.__class__.__name__, self.food))

　　#没有实现抽象方法时，实例化的时候不会报错，只有在调用的时候才会报错
    #def run(self):
        #pass
        
if __name__ == "__main__":
    Animal.register(Monkey)
    print (issubclass(Animal,Monkey))
    m = Monkey()
    m.eat()

 >>>> come in
>>>> True
>>>> Monkey eat banana
>>>> (<class '__main__.Monkey'>, <class 'object'>)

注册方式的缺点：不会出现在类的MRO (Method Resolution Order)，故而也不能通过super()来调用抽象方法。当没有实现抽象方法时，实例化时候不会报错，只有在调用时候才会报错。

继承方法

　　直接从抽象基类派生子类有一个好处，除非子类实现抽象基类的抽象方法，否则子类不能实例化。　

class Monkey(Animal):
    def __init__(self):
        super().__init__()
        self.food= "banana"
        
    def eat(self):
        print ("{0} eat {1}".format(self.__class__.__name__, self.food))

    def run(self):
        pass
        
if __name__ == "__main__":
    Animal.register(Monkey)
    print (issubclass(Animal,Monkey)))
    m = Monkey()
    m.eat()
    print (Monkey.__mro__)

3. ABCMeta类和ABC类源码

class ABCMeta(type):

    """Metaclass for defining Abstract Base Classes (ABCs).

    Use this metaclass to create an ABC.  An ABC can be subclassed
    directly, and then acts as a mix-in class.  You can also register
    unrelated concrete classes (even built-in classes) and unrelated
    ABCs as 'virtual subclasses' -- these and their descendants will
    be considered subclasses of the registering ABC by the built-in
    issubclass() function, but the registering ABC won't show up in
    their MRO (Method Resolution Order) nor will method
    implementations defined by the registering ABC be callable (not
    even via super()).

    """

    # A global counter that is incremented each time a class is
    # registered as a virtual subclass of anything.  It forces the
    # negative cache to be cleared before its next use.
    # Note: this counter is private. Use `abc.get_cache_token()` for
    #       external code.
    _abc_invalidation_counter = 0

    def __new__(mcls, name, bases, namespace):
        cls = super().__new__(mcls, name, bases, namespace)
        # Compute set of abstract method names
        abstracts = {name
                     for name, value in namespace.items()
                     if getattr(value, "__isabstractmethod__", False)}
        for base in bases:
            for name in getattr(base, "__abstractmethods__", set()):
                value = getattr(cls, name, None)
                if getattr(value, "__isabstractmethod__", False):
                    abstracts.add(name)
        cls.__abstractmethods__ = frozenset(abstracts)
        # Set up inheritance registry
        cls._abc_registry = WeakSet()
        cls._abc_cache = WeakSet()
        cls._abc_negative_cache = WeakSet()
        cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
        return cls

    def register(cls, subclass):
        """Register a virtual subclass of an ABC.

        Returns the subclass, to allow usage as a class decorator.
        """
        if not isinstance(subclass, type):
            raise TypeError("Can only register classes")
        if issubclass(subclass, cls):
            return subclass  # Already a subclass
        # Subtle: test for cycles *after* testing for "already a subclass";
        # this means we allow X.register(X) and interpret it as a no-op.
        if issubclass(cls, subclass):
            # This would create a cycle, which is bad for the algorithm below
            raise RuntimeError("Refusing to create an inheritance cycle")
        cls._abc_registry.add(subclass)
        ABCMeta._abc_invalidation_counter += 1  # Invalidate negative cache
        return subclass

    def _dump_registry(cls, file=None):
        """Debug helper to print the ABC registry."""
        print("Class: %s.%s" % (cls.__module__, cls.__qualname__), file=file)
        print("Inv.counter: %s" % ABCMeta._abc_invalidation_counter, file=file)
        for name in sorted(cls.__dict__.keys()):
            if name.startswith("_abc_"):
                value = getattr(cls, name)
                print("%s: %r" % (name, value), file=file)

    def __instancecheck__(cls, instance):
        """Override for isinstance(instance, cls)."""
        # Inline the cache checking
        subclass = instance.__class__
        if subclass in cls._abc_cache:
            return True
        subtype = type(instance)
        if subtype is subclass:
            if (cls._abc_negative_cache_version ==
                ABCMeta._abc_invalidation_counter and
                subclass in cls._abc_negative_cache):
                return False
            # Fall back to the subclass check.
            return cls.__subclasscheck__(subclass)
        return any(cls.__subclasscheck__(c) for c in {subclass, subtype})

    def __subclasscheck__(cls, subclass):
        """Override for issubclass(subclass, cls)."""
        # Check cache
        if subclass in cls._abc_cache:
            return True
        # Check negative cache; may have to invalidate
        if cls._abc_negative_cache_version < ABCMeta._abc_invalidation_counter:
            # Invalidate the negative cache
            cls._abc_negative_cache = WeakSet()
            cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
        elif subclass in cls._abc_negative_cache:
            return False
        # Check the subclass hook
        ok = cls.__subclasshook__(subclass)
        if ok is not NotImplemented:
            assert isinstance(ok, bool)
            if ok:
                cls._abc_cache.add(subclass)
            else:
                cls._abc_negative_cache.add(subclass)
            return ok
        # Check if it's a direct subclass
        if cls in getattr(subclass, '__mro__', ()):
            cls._abc_cache.add(subclass)
            return True
        # Check if it's a subclass of a registered class (recursive)
        for rcls in cls._abc_registry:
            if issubclass(subclass, rcls):
                cls._abc_cache.add(subclass)
                return True
        # Check if it's a subclass of a subclass (recursive)
        for scls in cls.__subclasses__():
            if issubclass(subclass, scls):
                cls._abc_cache.add(subclass)
                return True
        # No dice; update negative cache
        cls._abc_negative_cache.add(subclass)
        return False


class ABC(metaclass=ABCMeta):
    """Helper class that provides a standard way to create an ABC using
    inheritance.
    """
    pass        

4.Python中的抽象基类

　　collections.abc.Callable

　　collections.abc.Iterator

　　collections.abc.Mapping

　　numbers

　　以上为Python提供的常用抽象基类，若要了解各抽象基类的相关信息，请参考具体的文档