python元类深入理解

class A():
    def __init__(self,name):
        self.name = name
        print("创建了一个实例")

a = type("a",(A,),{"name":"jiao"}) 
print(a)              #<class ‘__main__.a‘>
print(a.name)          #jiao
print(a("jiang"))      #创建了一个实例  
                        #<__main__.a object at 0x00000280A973AA58>

class UpperAttrMetaClass(type):
    def __new__(cls,class_name,class_parents,class_attr, *args, **kwargs):
        print("__new__")
        class_attr[‘name‘] = "jiao"
        return type.__new__(cls,class_name,class_parents,class_attr)

    def __init__(self,*args,**kwargs):
        print("__init__")
        super().__init__(*args, **kwargs)
        self.__cache = {}

    def __call__(self, *args, **kwargs):
        print("__call__")
        if args in self.__cache:
            return self.__cache[args]
        else:
            obj = super().__call__(*args)
            self.__cache[args] = obj
            return obj


class A(metaclass=UpperAttrMetaClass):
    def __init__(self,name):
        self.name = name
        print("a.__init__")

class Singleton(type):
    def __init__(cls,*args,**kwargs):
        cls.__instance = None
        super().__init__(*args,**kwargs)

    def __call__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = super().__call__(*args,**kwargs)
            return cls.__instance
        else:
            return cls.__instance

class Spam(metaclass=Singleton):
    def __init__(self):
        print("Creating Spam")

import weakref

class Cached(type):
    def __init__(cls,*args,**kwargs):
        super().__init__(*args,**kwargs)
        cls.__cache = weakref.WeakValueDictionary()

    def __call__(cls, *args, **kwargs):
        if args in cls.__cache:
            return cls.__cache[args]
        else:
            obj = super().__call__(*args)
            cls.__cache[args] = obj
            return obj


class Spams(metaclass=Cached):
    def __init__(self,name):
        print("Creating Spam({!r})".format(name))
        self.name = name

from collections import OrderedDict

class Typed:
    _excepted_type = type(None)

    def __init__(self,name=None):
        self._name = name

    def __set__(self, instance, value):
        if not isinstance(value,self._excepted_type):
            raise TypeError("Excepted"+str(self._excepted_type))
        instance.__dict__[self._name] = value

class Integer(Typed):
    _excepted_type = int

class Float(Typed):
    _excepted_type = float

class String(Typed):
    _excepted_type = str

class OrderedMeta(type):

    def __new__(cls, clsname,bases,clsdict):
        d = dict(clsdict)
        order = []
        for name,value in clsdict.items():
            if isinstance(value,Typed):
                value._name = name
                order.append(name)
                d[‘_order‘] = order
       return type.__new__(cls,clsname,bases,d)

    @classmethod
    def __prepare__(metacls, name, bases):
        return OrderedDict()

#注：__prepare__该方法会在类定义一开始的时候调用，调用时以类名和基类名称作为参数，它必须返回一个映射对象，供处理类定义体时调用


#eg.
class Structure(metaclass=OrderedMeta):

    def as_csv(self):
        return ‘,‘.join(str(getattr(self,name)) for name in self._order)

class Stock(metaclass=OrderedMeta):
    name = String()
    shares = Integer()
    price = Float()
    def __init__(self,name,shares,price):
        self.name = name
        self.shares = shares
        self.price = price

s = Stock("haha",23,23.3)
print(s.name)
s = Stock(34,23,34)
# print(s.as_csv())