gj8 元类编程

水深无声 2021-12-14 13:29 294阅读 0赞

## 8.1 property动态属性 ##

from datetime import date, 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    # 将函数变成属性的方式访问，可以通过 user.age 访问 
        def age(self):
            return datetime.now().year - self.birthday.year
    
        @age.setter  # 设置属性  user.age = 24
        def age(self, value):
            self._age = value
    if __name__ == "__main__":
        user = User("bobby", date(year=1987, month=1, day=1))
        user.age = 30
        print (user._age)
        print(user.age)
    # ---
    30
    32

## 8.2 \_\_getattr\_\_、\_\_getattribute\_\_魔法函数 ##

# __getattr__, __getattribute__
    # __getattr__ 就是在查找不到属性的时候调用
    
    class User:
        def __init__(self, info={}):
            self.info = info
    
        def __getattr__(self, item):  # 属性不存在的时候，进入
            return self.info[item]
            # return "not find "
        # def __getattribute__(self, item):   # 无条件的进入，所有属性的访问入口
        #     return "lewen"
    
    
    if __name__ == "__main__":
        user = User(info={"company_name": "imooc", "name": "lewen"})
        print(user.name)

---

lewen

## 8.3 属性描述符和属性查找过程 ##

import numbers
    
    class IntField:
        # 数据属性描述符，任意实现下面一个就是
        def __get__(self, instance, owner):  #
            return self.value
    
        def __set__(self, instance, value):
            if not isinstance(value, numbers.Integral):  # 参数类型检查
                raise ValueError("int value need")
            if value < 0:
                raise ValueError("positive value need")
            self.value = value
    
        def __delete__(self, instance):
            pass
    
    class User:
        age = IntField()
    
    if __name__ == "__main__":
        user = User()
        user.age = 30
    #     user.__dict__["age"] = "abc"   #  print(user.age) 会报错，找不到value
        print(user.__dict__) # {} 并没有进入对象实例
        print(user.age)
        print (getattr(user, 'age'))
    # ---
    {}
    30
    30
    # ---
    
    
    
    class NonDataIntField:
        # 非数据属性描述符
        def __get__(self, instance, owner):
            return self.value
    class User:
        age = NonDataIntField()
    if __name__ == "__main__":
        user = User()
        user.age = 30
        print(user.__dict__) # {'age': 30}
        print(user.age)
        print (getattr(user, 'age'))
    
    
    # ===
    {'age': 30}
    30
    30
    # ------
    
    
    '''
    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，如果age是non-data descriptor，那么调用其__get__方法， 否则返回 __dict__[‘age’]
    （4）如果User有__getattr__方法，调用__getattr__方法，否则
    （5）抛出AttributeError
    
    '''

## 8.4 \_\_new\_\_和\_\_init\_\_的区别 ##

class User:
        def __new__(cls, *args, **kwargs):
            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")
    
    ---
     in new
     in init

## 8.5 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
    if __name__ == "__main__":
        MyClass = create_class("user")
        my_obj = MyClass()
        print(type(my_obj))
    ---
    <class '__main__.create_class.<locals>.User'>
    
    
    #type动态创建类
    
    def say(self):               # 定义的方法
        return "i am user"
        # return self.name
    
    class BaseClass():           # 基类
        def answer(self):
            return "i am baseclass"
    
    User = type("User", (BaseClass,), {"name":"lewen","say":say})
    #           类名   模板    属性
    my_obj = User()
    print(my_obj.name)
    print(my_obj.say())   # 调用方法
    print(my_obj.answer()) # 调用基类方法

# ---
    lewen
    i am user
    i am baseclass
    ---

#什么是元类， 元类是创建类的类 对象<-class(对象)<-type
    from collections.abc import *
    class MetaClass(type):                      # 控制实例化的过程
        def __new__(cls, *args, **kwargs):      # 作初始检查
            return super().__new__(cls, *args, **kwargs)
    
    class User(metaclass=MetaClass):
        def __init__(self, name):
            self.name = name
        def __str__(self):
            return "user:%s"%(self.name)
    
    # python中类的实例化过程，会首先寻找metaclass，通过metaclass去创建user类.
    # 没有继承的metaclass找到 则最后由 
    # type去创建类对象，实例
    
    my_obj = User("lewen")
    print(my_obj)

---
    user:lewen

## 8.6 元类实现简单的orm ##

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":  # 反回 BaseModel 类
                return super().__new__(cls, name, bases, attrs, **kwargs)
    
            # 子类的数据封装
            fields = {}
            for key, value in attrs.items():
                if isinstance(value, Field):  # 封装属性字段值
                    fields[key] = value
            attrs_meta = attrs.get("Meta", None)  # 获取User类中的 Meta 类对象
            _meta = {}
            db_table = name.lower()  # 默认表名
            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  # 设置表名
    
            attrs["_meta"] = _meta
            attrs["fields"] = fields
            del attrs["Meta"]
            return super().__new__(cls, name, bases, attrs, **kwargs)
    
    
    # 先__new__将创建类的数据进行封装并返回类对象 ,再__init__初始化
    class BaseModel(metaclass=ModelMetaClass):
        # 处理子类实例化时初始值
        def __init__(self, *args, **kwargs):
            for key, value in kwargs.items():
                setattr(self, key, value)
            return super().__init__()        #？
            # 反回__init__方法，子类中不需要定义
    
        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):  # 创建类触发__new__： 类名User, bases 为BaseModel，下面的是属性 attrs
    
        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="lewen", age=28)  # 实例化，触发__init__
        # user.name = "bobby"
        # user.age = 28
        user.save()

[![image][]][image 1]

[![image][image 2]][image_image 2]

转载于:https://www.cnblogs.com/wenyule/p/10366646.html

[image]: /images/20211214/fa01144136854c4890e9b34e1640ebdf.png
[image 1]: https://img2018.cnblogs.com/blog/1165731/201902/1165731-20190212182802229-1546473301.png
[image 2]: https://img2018.cnblogs.com/blog/1165731/201902/1165731-20190212182806684-189706850.png
[image_image 2]: /images/20211214/5ee5ac24a899429ba674da717365226b.png