python 元类的使用

class Animal(object):
    """a class representing an animal"""
    def __init__(self,name):
        self.name = name

    def eat(self):
        pass

    def go_to_vet(self):
        pass

animal = Animal('cat')
print(type(Animal))
print(animal)

#用type方式创建类******************************************************************
def init(self,name):
    self.name = name

def eat(self):
    pass

def go_to_vet(self):
    pass

Animal = type('Animal',(object,),
              {'__doc__':"a class representing an animal",'__init__':init,
                'eat':eat,'go_to_vet':go_to_vet})

animal = Animal('dog')
print(animal)

#创建子类
def meow(self):
    return None

def purr(self):
    return None



Cat = type('Cat',(Animal,),{
    'meow':meow,
    'purr':purr
})

cat = Cat(name='xiaohuang')
print(cat)

print(type(Cat))

#元类示例******************************************************************
class Meta(type):
    """a metaclass that adds no actual functionality"""
    def _(cls,name,bases,attrs):
        """

        :param name: 类型：字符串 类名
        :param bases: 类型：元组，继承的基类
        :param attrs:  类型：字典  属性
        :return: 类
        """
        return super(Meta, cls).__new__(cls,name,bases,attrs)


#何时使用元类******************************************************************
# 1.说明性类声明，现有示例django模型声明

# 2.类验证 (foo 和 bar属性两个只能包含一个)
class FooOrbar(type):

    def __new__(cls, name,bases,attrs):
        if 'foo' in attrs and 'bar' in attrs:
            raise TypeError('Class %s cannot contain both foo and bar'%name)
        if 'foo' not in attrs and 'bar' not in attrs:
            raise TypeError('Class %s must provide either  foo or bar'%name)
        return super(FooOrbar,cls).__new__(cls,name,bases,attrs)


class Valid(metaclass=FooOrbar):
    foo = 42

v = Valid()

class Invalid(metaclass=FooOrbar):
    foo =41


# 以上实现存在一个问题，他的子类并不会继承该功能，原因是元类直接检查attrs属性，但这只包含所声明的类属性集，
# 他并不知道继承自基类的属性

# class AlsoValid(Valid):
#     pass


# 改进如下
class FooOrBarUpdate(type):
    def __new__(cls, name,bases,attrs):
        """实例化，从基类中获取了所有的属性"""
        answer = super(FooOrBarUpdate, cls).__new__(cls,name,bases,attrs)

        if hasattr(answer,'foo') and hasattr(answer,'bar'):
            raise TypeError('Class %s cannot contain both foo and bar'%name)

        if not hasattr(answer,'foo') and not hasattr(answer,'bar'):
            raise TypeError('Class %s must provide either  foo or bar' % name)

        return answer

# 3.非继承性

class Meta1(type):
    def __new__(cls, name,bases,attrs):
        if attrs.get('abstract',False):
            return super(Meta1,cls).__new__(cls,name,bases,attrs)

class SubClass(metaclass=Meta1):
    abstract = False

# 以上元类定义存在一个问题，任何子类都需要显示的声明自己并不是抽象类
# 改进如下 ：
class MetaUpdate1(type):

    def __new__(cls, name,bases,attrs):
        if attrs.pop('abstract',False):
            return super(MetaUpdate1, cls).__new__(cls,name,bases,attrs)

#meta-coding***************************************************************

class Logged(type):

    """"a meta class that cause classes that it creates to log their function calls"""
    def __new__(cls, name,bases,attrs):
        for k,v in attrs.items():
            if callable(v):
                attrs[k] = cls.log_call(v)

        return super(Logged, cls).__new__(cls,name,bases,attrs)

    @staticmethod
    def log_call(v):

        def inner(*args,**kwargs):

            try:
                response = v(*args,**kwargs)
                print('%s函数被调用，参数为%s，字典关键字为:%s.'%(v.__name__,args,kwargs))
                return response
            except Exception as e:
                print('%s函数被调用，抛出异常：%s'%(v.__name__,e))
                raise e

        return inner

class MyClass(metaclass=Logged):

    def foo(self):
        pass

    def bar(self):
        raise TypeError('oh,noes')

obj = MyClass()
obj.foo()
obj.bar()