• 第七章:Python高级编程-元类编程


    第七章:Python高级编程-元类编程

    Python3高级核心技术97讲 笔记

    7.1 property动态属性

    from datetime import data, datetime
    
    class User:
        def __init__(self, name, birthday):
            self.name = name
            self.birthday = birthday
            self._age = 0
            
    	def get_age(self):
            return datetime.now().year - self.birthday.year
        
        
        @property
        def age(self):
            return datetime.now().year - self.birthday.year
        
        
        @age.setter
        def age(self, value):
            self._age = value
            
    if __name__ == "__main__":
        user = User("bobby", date(year=1987, month=1, day=1))
        print(user.get_age())
        user.age = 30
        print(self._age)
    

    7.2 __getattr__、__getattribute__魔法函数

    #__getattr__, __getattribute__
    #__getattr__ 就是在查找不到属性的时候调用
    from datetime import date
    class User:
        def __init__(self,info={}):
            self.info = info
    
        def __getattr__(self, item):  # 查找不到属性的时候调用
            return self.info[item]
    
        # def __getattribute__(self, item):  # 查找属性时调用
        #     return "bobby"
    
    if __name__ == "__main__":
        user = User(info={"company_name":"imooc", "name":"bobby"})
        print(user.test)
    

    7.3 属性描述符和属性查找过程

    from datetime import date, datetime
    import numbers
    
    class IntField:
        #数据描述符
        def __get__(self, instance, owner):
            return self.value
        def __set__(self, instance, value):
            if not isinstance(value, numbers.Integral):
                raise ValueError("int value need")
            if value < 0:
                raise ValueError("positive value need")
            self.value = value
        def __delete__(self, instance):
            pass
    
    
    class NonDataIntField:
        #非数据属性描述符
        def __get__(self, instance, owner):
            return self.value
    
    class User:
        age = IntField()
        # age = NonDataIntField()
    
    '''
    如果user是某个类的实例,那么user.age(以及等价的getattr(user,’age’))
    首先调用__getattribute__。如果类定义了__getattr__方法,
    那么在__getattribute__抛出 AttributeError 的时候就会调用到__getattr__,
    而对于描述符(__get__)的调用,则是发生在__getattribute__内部的。
    user = User(), 那么user.age 顺序如下:
    
    (1)如果“age”是出现在User或其基类的__dict__中, 且age是data descriptor, 那么调用其__get__方法, 否则
    
    (2)如果“age”出现在user的__dict__中, 那么直接返回 obj.__dict__[‘age’], 否则
    
    (3)如果“age”出现在User或其基类的__dict__中
    
    (3.1)如果age是non-data descriptor,那么调用其__get__方法, 否则
    
    (3.2)返回 __dict__[‘age’]
    
    (4)如果User有__getattr__方法,调用__getattr__方法,否则
    
    (5)抛出AttributeError
    
    '''
    
    # class User:
    #
    #     def __init__(self, name, email, birthday):
    #         self.name = name
    #         self.email = email
    #         self.birthday = birthday
    #         self._age = 0
    #
    #     # def get_age(self):
    #     #     return datetime.now().year - self.birthday.year
    #
    #     @property
    #     def age(self):
    #         return datetime.now().year - self.birthday.year
    #
    #     @age.setter
    #     def age(self, value):
    #         #检查是否是字符串类型
    #         self._age = value
    
    if __name__ == "__main__":
        user = User()
        user.__dict__["age"] = "abc"
        print (user.__dict__)
        print (user.age)
        # print (getattr(user, 'age'))
        # user = User("bobby", date(year=1987, month=1, day=1))
        # user.age = 30
        # print (user._age)
        # print(user.age)
    
    
    

    7.4 __new__和__init__的区别

    class User:
        def __new__(cls, *args, **kwargs):  # args 位置参数  kwargs 有名参数
            print (" in new ")
            return super().__new__(cls)
        def __init__(self, name):
            print (" in init")
            pass
    a = int()
    #new 是用来控制对象的生成过程, 在对象生成之前
    #init是用来完善对象的
    #如果new方法不返回对象, 则不会调用init函数
    if __name__ == "__main__":
        user = User(name="bobby")
    
    

    7.5 自定义元类

    #类也是对象,type创建类的类
    def create_class(name):
        if name == "user":
            class User:
                def __str__(self):
                    return "user"
            return User
        elif name == "company":
            class Company:
                def __str__(self):
                    return "company"
            return Company
    
    #type动态创建类
    # User = type("User", (), {})
    
    def say(self):
        return "i am user"
        # return self.name
    
    
    class BaseClass():
        def answer(self):
            return "i am baseclass"
    
    
    class MetaClass(type):
        def __new__(cls, *args, **kwargs):
            return super().__new__(cls, *args, **kwargs)
    
    from collections.abc import *
    
    #什么是元类, 元类是创建类的类 对象<-class(对象)<-type
    class User(metaclass=MetaClass):
        def __init__(self, name):
            self.name = name
        def __str__(self):
            return "user"
    #python中类的实例化过程,会首先寻找metaclass,通过metaclass去创建user类
    #去创建类对象,实例
    
    if __name__ == "__main__":
        # MyClass = create_class("user")
        # my_obj = MyClass()
        # print(type(my_obj))
    
        # User = type("User", (BaseClass, ), {"name":"user", "say":say})
        my_obj = User(name="bobby")
        print(my_obj)
    
    

    7.6 自定义元类

    # 需求
    import numbers
    
    
    class Field:
        pass
    
    class IntField(Field):
        # 数据描述符
        def __init__(self, db_column, min_value=None, max_value=None):
            self._value = None
            self.min_value = min_value
            self.max_value = max_value
            self.db_column = db_column
            if min_value is not None:
                if not isinstance(min_value, numbers.Integral):
                    raise ValueError("min_value must be int")
                elif min_value < 0:
                    raise ValueError("min_value must be positive int")
            if max_value is not None:
                if not isinstance(max_value, numbers.Integral):
                    raise ValueError("max_value must be int")
                elif max_value < 0:
                    raise ValueError("max_value must be positive int")
            if min_value is not None and max_value is not None:
                if min_value > max_value:
                    raise ValueError("min_value must be smaller than max_value")
    
        def __get__(self, instance, owner):
            return self._value
    
        def __set__(self, instance, value):
            if not isinstance(value, numbers.Integral):
                raise ValueError("int value need")
            if value < self.min_value or value > self.max_value:
                raise ValueError("value must between min_value and max_value")
            self._value = value
    
    
    class CharField(Field):
        def __init__(self, db_column, max_length=None):
            self._value = None
            self.db_column = db_column
            if max_length is None:
                raise ValueError("you must spcify max_lenth for charfiled")
            self.max_length = max_length
    
        def __get__(self, instance, owner):
            return self._value
    
        def __set__(self, instance, value):
            if not isinstance(value, str):
                raise ValueError("string value need")
            if len(value) > self.max_length:
                raise ValueError("value len excess len of max_length")
            self._value = value
    
    
    class ModelMetaClass(type):
        def __new__(cls, name, bases, attrs, **kwargs):
            if name == "BaseModel":
                return super().__new__(cls, name, bases, attrs, **kwargs)
            fields = {}
            for key, value in attrs.items():
                if isinstance(v alue, Field):
                    fields[key] = value
            attrs_meta = attrs.get("Meta", None)
            _meta = {}
            db_table = name.lower()
            if attrs_meta is not None:
                table = getattr(attrs_meta, "db_table", None)
                if table is not None:
                    db_table = table
            _meta["db_table"] = db_table
            attrs["_meta"] = _meta
            attrs["fields"] = fields
            del attrs["Meta"]
            return super().__new__(cls, name, bases, attrs, **kwargs)
    
    
    class BaseModel(metaclass=ModelMetaClass):
        def __init__(self, *args, **kwargs):
            for key, value in kwargs.items():
                setattr(self, key, value)
            return super().__init__()
    
        def save(self):
            fields = []
            values = []
            for key, value in self.fields.items():
                db_column = value.db_column
                if db_column is None:
                    db_column = key.lower()
                fields.append(db_column)
                value = getattr(self, key)
                values.append(str(value))
    
            sql = "insert {db_table}({fields}) value({values})".format(db_table=self._meta["db_table"],
                                                                       fields=",".join(fields), values=",".join(values))
            pass
    
    class User(BaseModel):
        name = CharField(db_column="name", max_length=10)
        age = IntField(db_column="age", min_value=1, max_value=100)
    
        class Meta:
            db_table = "user"
    
    
    if __name__ == "__main__":
        user = User(name="bobby", age=28)
        # user.name = "bobby"
        # user.age = 28
        user.save()
    
    
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  • 原文地址:https://www.cnblogs.com/xunjishu/p/12857691.html
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