python中通过元类(TYPE)简单实现对象关系映射(ORM)

class User(父类):
    uid = ("uid", "int unsigned")
    name = ("username", "varchar(20)")
    password = ("password", "varchar(20)")
    ...省略...

u = User(nid=12345, name="laowang",password="123321")

insert into (uid,name,password) value (12345,"laowang","123321")

class Mode_type(type):
    def __new__(cls, name, bases, attrs)
        mappings = dict()
        for k, v in attrs.items():
            if isinstanse(v, tuple):
                mappings[k] = v
    
        # 找到新字典接收完attrs的数据后，删掉attrs里的数据
        for k in mappings.keys[]:
            attrs.pop(k)
        
        # 将之前新字典保存的数据表的信息保存在attrs中
        attrs["__mappings__"] = mappings
        # name指向新创建的实例对象名，相当于保存数据表的名称
        attrs["__table__"] = name    
        return type.__new__(cls, name, bases, attrs)


class User(mateclass = Mode_type):
    uid = ("uid", "int unsigned")
    name = ("username", "varchar(20)")
    password = ("password", "varchar(20)")
    #  指定元类后，以上的类属性就不在类中，而是在__mapping__指定的字典中仓储
    # 类似于
    # __mapping__ = {
    #        uid :("uid", "int unsigned")
    #        name: ("username", "varchar(20)")
    #        password :("password", "varchar(20)")
    # }
    # __table__ = "User"

    def __init__(self, **kwargs):
        # 取出字典里面的值
        for name, value in kwargs.items():
            setattr(self, name, value)
            # 这里不能用self.name = value ，这样只会让实例对象拥有name这个属性，而不是name形参背后真正替换的属性，所以用setattr

    def save()
        # 数据表表头字段
        fields = []
        # 数据表字段对应的数据
        args = []
        for k, v in self.__mappings__.items():
            field.append(v[0])
            args.append(getattr(self, k, None))
        args_temp = list()
        # 区分参数的类型，防止写入数据表后报错
        for temp in args:
            if isinstence(temp, int):
                args_temp.append(str(temp))  # 类似temp = “123456”
            elif isinstence(temp, str)：
                args_temp.append("""%s""" % temp)  # 类似temp = """‘123321‘"""
        sql = "insert into %s (%s) value (%s)" % (self.__table__, ",".join(fields), ",".join(args_temp))
        #  下面就可以执行mysql的操作，只是说ORM，所以我只打印了这句话    
        print(sql)


u = User(uid = 123456, name="laowang"， password = "123321"）
u.save()