Python之__new__

__new__为Python中的创建实例函数(在实例化类时,开辟一个空间)

类加括号实例化对象的步骤

1, 创建一个该类的空对象   __new__

2, 实例化该空对象  __init__

3, 将实例化的空对象返回  return cls

 

重写__new__

1. 该方法是从type中继承来的,所以参数同type的init

2. 最终的工作(如果开辟空间,如果操作内存)还是要借助type

3. 再交给type最终完成工作之前,可以对垒的创建加以限制

class MyMetaClass(type):   # 要重写init必须继承type
    def __new__(self, class_name, class_bases, class_addrs):
        print(class_name)  # 类名
        print(class_bases)  # 继承的基类
        print(class_addrs)  # 名称空间
　　　　　super().__init__(class_name, class_bases, class_addrs)  # 最终还是要借助type


class Foo(metaclass=MyMetaClass): def __init__(self): pass obj = Foo() # Foo = MyMetaClass(class_name, class_bases, class_addrs)

'''

　　Foo
　　()
　　{'__module__': '__main__', '__qualname__': 'Foo', '__init__': <function Foo.__init__ at 0x032D8738>}

'''

案例一

from connect_mysql import Mysql


class Field(object):
    def __init__(self,name, column_type, primary_key, default):
        self.name = name
        self.column_type = column_type
        self.primary_key = primary_key
        self.default = default


class StringField(Field):
    def __init__(self,name, column_type='varchar(255)', primary_key=False, default=None):
        super().__init__(name,column_type,primary_key,default)


class IntegerField(Field):
    def __init__(self,name, column_type='int', primary_key=False, default=None):
        super().__init__(name,column_type,primary_key,default)


class MyMetaClass(type):
    def __new__(cls, class_name, class_bases, class_addrs):
        if class_name == 'Models': return type.__new__(cls, class_name, class_bases, class_addrs)
        table_name = class_addrs.get('table_name', class_name)
        primary_key = None
        mappings = {}
        for k,v in class_addrs.items():
            if isinstance(v, Field):
                mappings[k] = v
                if v.primary_key:
                    if primary_key:
                        raise TypeError('最多只能拥有一个主键!!!')
                    primary_key = k
        if not primary_key:
            raise TypeError('必须拥有一个主键!!!')
        for k in mappings.keys():
            class_addrs.pop(k)
        class_addrs['primary_key'] = primary_key
        class_addrs['mappings'] = mappings
        class_addrs['table_name'] = table_name
        return type.__new__(cls, class_name, class_bases, class_addrs)


class Models(dict, metaclass=MyMetaClass):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

    def __getattr__(self, item):
        return self.get(item, '暂无此键!!!')

    def __setattr__(self, key, value):
        self[key] = value

    @classmethod
    def select(cls, **kwargs):
        conn = Mysql()
        if not kwargs:
            sql = 'select * from %s'%cls.table_name
            res = conn.select(sql)
        else:
            key = list(kwargs.keys())[0]
            value = kwargs[key]
            sql = 'select * from %s where %s = ?'%(cls.table_name, key)
            sql = sql.replace('?','%s')
            res = conn.select(sql, value)
        print(sql)
        if res:
            return [cls(**r) for r in res]

    def update(self):
        # sql = 'update user set name='dsada',age='dasd' where id = '1''
        conn = Mysql()
        primary_key = None
        keys = []
        values = []
        for k,v in self.mappings.items():
            if v.primary_key:
                primary_key = getattr(self, v.name, v.default)
            else:
                keys.append(v.name + '=?')
                values.append(getattr(self, v.name, v.default))
        sql = 'update %s set %s where %s = %s'%(self.table_name, ','.join(keys), self.primary_key, primary_key)
        sql = sql.replace('?', '%s')
        print(sql)
        print(values)
        conn.execute(sql, values)

    def save(self):
        # sql = 'insert into user(name,age) value ('dasd','dada)'
        conn = Mysql()
        keys = []
        values = []
        args = []
        for k,v in self.mappings.items():
            if not v.primary_key:
                keys.append(v.name)
                values.append(getattr(self, v.name, v.default))
                args.append('?')
        sql = 'insert into %s(%s) value (%s)'%(self.table_name, ','.join(keys), ','.join(args))
        print(sql)
        print(values)
        sql = sql.replace('?', '%s')
        conn.execute(sql, values)



if __name__ == '__main__':
    class User(Models):
        id = IntegerField(name='id',primary_key=True)
        name = StringField(name='name')
        password = StringField(name='password')
        is_vip = IntegerField(name='is_vip')
        is_locked = IntegerField(name='is_locked')
        user_type = StringField(name='user_type')
        register_time = StringField(name='register_time')

    obj = User(name='dasd',password='dasd',is_vip=0,is_locked=0,user_type='admin',register_time='2019-05-27 09:42:33')
    obj.save()