day34 元类和单例模式

Python元类深入解析

最新推荐文章于 2025-03-18 17:40:22 发布

原创最新推荐文章于 2025-03-18 17:40:22 发布 · 134 阅读

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一切源自于一句话：python中一切皆为对象

一：元类介绍

元类=》OldboyTeacher类=》obj

class OldboyTeacher(object):
    school = 'oldboy'

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def say(self):
        print('%s says welcome to the oldboy to learn Python' % self.name)


obj = OldboyTeacher('egon', 18)  # 调用OldboyTeacher类=》对象obj
#                                  调用元类=》OldboyTeacher类

# print(type(obj))
print(type(OldboyTeacher))

结论：默认的元类是type，默认情况下我们用class关键字定义的类都是由type产生的

二：class关键字底层的做了哪些事

1、先拿到一个类名

class_name = "OldboyTeacher"

2、然后拿到类的父类

class_bases = (object,)

3、再运行类体代码，将产生的名字放到名称空间中

class_dic = {}
class_body = """
school = 'oldboy'

def __init__(self, name, age):
    self.name = name
    self.age = age

def say(self):
    print('%s says welcome to the oldboy to learn Python' % self.name)
"""
exec(class_body,{},class_dic)
# print(class_dic)

4、调用元类（传入类的三大要素：类名、基类、类的名称空间）得到一个元类的对象，然后将元类的对象赋值给变量名OldboyTeacher,oldboyTeacher就是我们用class自定义的那个类

OldboyTeacher = type(class_name,class_bases,class_dic)

三：自定义元类

class Mymeta(type):  # 只有继承了type类的类才是自定义的元类
    pass


class OldboyTeacher(object, metaclass=Mymeta):
    school = 'oldboy'

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def say(self):
        print('%s says welcome to the oldboy to learn Python' % self.name)

1、先拿到一个类名:

`"OldboyTeacher"`

2、然后拿到类的父类:
(object,)
3、再运行类体代码，将产生的名字放到名称空间中{...}
4、调用元类（传入类的三大要素：类名、基类、类的名称空间）得到一个元类的对象，然后将元类的对象赋值给变量名OldboyTeacher,oldboyTeacher就是我们用class自定义的那个类
OldboyTeacher = Mymeta("OldboyTeacher",(object,),{...})
'''

四：自定义元类来控制OldboyTeacher类的产生

import re


class Mymeta(type):  # 只有继承了type类的类才是自定义的元类
    def __init__(self, class_name, class_bases, class_dic):
        # print(self)  # 类<class '__main__.OldboyTeacher'>
        # print(class_name)
        # print(class_bases)
        # print(class_dic)

        if not re.match("[A-Z]", class_name):
            raise BaseException("类名必须用驼峰体")

        if len(class_bases) == 0:
            raise BaseException("至少继承一个父类")

        # print("文档注释：",class_dic.get('__doc__'))
        doc=class_dic.get('__doc__')

        if not (doc and len(doc.strip()) > 0):
            raise BaseException("必须要有文件注释，并且注释内容不为空")

# OldboyTeacher = Mymeta("OldboyTeacher",(object,),{...})
class OldboyTeacher(object,metaclass=Mymeta):
    """
    adsaf
    """

    school = 'oldboy'

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def say(self):
        print('%s says welcome to the oldboy to learn Python' % self.name)

'''

五：自定义元类来控制OldboyTeacher类的调用

import re


class Mymeta(type):  # 只有继承了type类的类才是自定义的元类
    def __init__(self, class_name, class_bases, class_dic):
        # print(self)  # 类<class '__main__.OldboyTeacher'>
        # print(class_name)
        # print(class_bases)
        # print(class_dic)

        if not re.match("[A-Z]", class_name):
            raise BaseException("类名必须用驼峰体")

        if len(class_bases) == 0:
            raise BaseException("至少继承一个父类")

        # print("文档注释：",class_dic.get('__doc__'))
        doc = class_dic.get('__doc__')

        if not (doc and len(doc.strip()) > 0):
            raise BaseException("必须要有文件注释，并且注释内容不为空")

    # res = OldboyTeacher('egon',18)
    def __call__(self, *args, **kwargs):
        # 1、先创建一个老师的空对象
        tea_obj = object.__new__(self)
        # 2、调用老师类内的__init__函数，然后将老师的空对象连同括号内的参数的参数一同传给__init__
        self.__init__(tea_obj, *args, **kwargs)
        tea_obj.__dict__ = {"_%s__%s" %(self.__name__,k): v for k, v in tea_obj.__dict__.items()}

        # 3、将初始化好的老师对象赋值给变量名res
        return tea_obj


# OldboyTeacher = Mymeta("OldboyTeacher",(object,),{...})
class OldboyTeacher(object, metaclass=Mymeta):
    """
    adsaf
    """

    school = 'oldboy'

    #            tea_obj,'egon',18
    def __init__(self, name, age):
        self.name = name  # tea_obj.name='egon'
        self.age = age  # tea_obj.age=18

    def say(self):
        print('%s says welcome to the oldboy to learn Python' % self.name)


res = OldboyTeacher('egon', 18)
print(res.__dict__)
# print(res.name)
# print(res.age)
# print(res.say)

调用OldboyTeacher类做的事情：
1、先创建一个老师的空对象
2、调用老师类内的__init__方法，然后将老师的空对象连同括号内的参数的参数一同传给__init__
3、将初始化好的老师对象赋值给变量名res

六：单例模式

实现方式1：classmethod


import settings


class MySQL:
    __instance = None

    def __init__(self, ip, port):
        self.ip = ip
        self.port = port

    @classmethod
    def singleton(cls):
        if cls.__instance:
            return cls.__instance
        cls.__instance = cls(settings.IP, settings.PORT)
        return cls.__instance


# obj1=MySQL("1.1.1.1",3306)
# obj2=MySQL("1.1.1.2",3306)
# print(obj1)
# print(obj2)


obj3 = MySQL.singleton()
print(obj3)

obj4 = MySQL.singleton()
print(obj4)

方式2：元类


import settings


class Mymeta(type):
    __instance = None
    def __init__(self,class_name,class_bases,class_dic):
        self.__instance=object.__new__(self)  # Mysql类的对象
        self.__init__(self.__instance,settings.IP,settings.PORT)

    def __call__(self, *args, **kwargs):
        if args or kwargs:
            obj = object.__new__(self)
            self.__init__(obj, *args, **kwargs)
            return obj
        else:
            return self.__instance

# MySQL=Mymeta(...)
class MySQL(metaclass=Mymeta):
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port


# obj1 = MySQL("1.1.1.1", 3306)
# obj2 = MySQL("1.1.1.2", 3306)
# print(obj1)
# print(obj2)

obj3 = MySQL()
obj4 = MySQL()

print(obj3 is obj4)

方式3：装饰器


import settings

def outter(func):  # func = MySQl类的内存地址
    _instance = func(settings.IP,settings.PORT)
    def wrapper(*args,**kwargs):
        if args or kwargs:
            res=func(*args,**kwargs)
            return res
        else:
            return _instance
    return wrapper

@outter  # MySQL=outter(MySQl类的内存地址)  # MySQL=》wrapper
class MySQL:
    def __init__(self, ip, port):
        self.ip = ip
        self.port = port


# obj1 = MySQL("1.1.1.1", 3306)
# obj2 = MySQL("1.1.1.2", 3306)
# print(obj1)
# print(obj2)

obj3 = MySQL()
obj4 = MySQL()
print(obj3 is obj4)

了解：属性查找

class Mymeta(type):
    n=444

    # def __call__(self, *args, **kwargs): #self=<class '__main__.OldboyTeacher'>
    #     obj=self.__new__(self)
    #     print(self.__new__ is object.__new__) #True


class Bar(object):
    # n=333

    # def __new__(cls, *args, **kwargs):
    #     print('Bar.__new__')
    pass

class Foo(Bar):
    # n=222

    # def __new__(cls, *args, **kwargs):
    #     print('Foo.__new__')
    pass

class OldboyTeacher(Foo,metaclass=Mymeta):
    # n=111

    school='oldboy'

    def __init__(self,name,age):
        # self.n=0
        self.name=name
        self.age=age

    def say(self):
        print('%s says welcome to the oldboy to learn Python' %self.name)


    # def __new__(cls, *args, **kwargs):
    #     print('OldboyTeacher.__new__')


# obj=OldboyTeacher('egon',18)
# print(obj.n)

print(OldboyTeacher.n)