面向对象中有哪些双下线方法及应用场景

#(1)__init__
#(2)__str__
#(3)__repr__

#(4)__new__：执行__init__之前执行__new__方法，在单例/rest framework序列化中使用过
        #在rest framework序列化源码中的使用
        class BaseSerializer(Field):
            def __init__(self, instance=None, data=empty, **kwargs):
                self.instance = instance
                if data is not empty:
                    self.initial_data = data
                self.partial = kwargs.pop(‘partial‘, False)
                self._context = kwargs.pop(‘context‘, {})
                kwargs.pop(‘many‘, None)
                super().__init__(**kwargs)

            def __new__(cls, *args, **kwargs):
                # We override this method in order to automagically create
                # `ListSerializer` classes instead when `many=True` is set.
                if kwargs.pop(‘many‘, False):
                    return cls.many_init(*args, **kwargs)
                return super().__new__(cls, *args, **kwargs)
                
#(5)__call__：flask源码请求入口，django请求入口（WSGIHandler.__call__）。
        #在flask源码请求入口中的使用
        class Flask(_PackageBoundObject):
            def __call__(self, environ, start_response):
            return self.wsgi_app(environ, start_response)
            
        #在django源码请求入口中的使用  
        class WSGIHandler(base.BaseHandler):
            request_class = WSGIRequest

            def __init__(self, *args, **kwargs):
                super().__init__(*args, **kwargs)
                self.load_middleware()

            def __call__(self, environ, start_response):
                set_script_prefix(get_script_name(environ))
                signals.request_started.send(sender=self.__class__, environ=environ)
                request = self.request_class(environ)
                response = self.get_response(request)
                #……

#(6)__getattr__
#(7)__setattr__
#(8)__delattr__：在flask源码Local对象中使用了这三个，见最下方

#(9)__setitem__
#(10)__getitem__
#(11)__delitem__ ：对象[‘xx‘]会触发这个三个
    class Foo(object):

        def __getitem__(self, item):
            return 1

        def __setitem__(self, key, value):
            pass
            
        def __delitem__(self, key):
            pass
            
    obj = Foo()
    obj[‘k1‘]           #触发__getitem__
    obj[‘k1‘] = 123     #触发__setitem__
    del obj[‘k1‘]       #触发__delitem__

#(12)__dict__，api封装返回数据时：BaseResponse
#(13)__mro__， 继承顺序
#(14)__slots__，Local对象

#flask源码Local对象
class Local(object):
    __slots__ = (‘__storage__‘, ‘__ident_func__‘)  #__slots__中设置的值就是外部能访问的值

    def __init__(self):
        # __storage__ = {1231:{‘stack‘:[]}}
        object.__setattr__(self, ‘__storage__‘, {})
        object.__setattr__(self, ‘__ident_func__‘, get_ident)

    def __getattr__(self, name):
        try:
            return self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)

    def __setattr__(self, name, value):
        ident = self.__ident_func__()  #相当于ident = get_ident()
        storage = self.__storage__     #相当于storge = {}
        try:
            storage[ident][name] = value
        except KeyError:
            storage[ident] = {name: value}  #唯一标识不存在情况下就进行设置

    def __delattr__(self, name):
        try:
            del self.__storage__[self.__ident_func__()][name]
        except KeyError:
            raise AttributeError(name)