Python-元类

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参考自本教程中的元类一篇，注释是自己的理解。

http://www.liaoxuefeng.com/wiki/0014316089557264a6b348958f449949df42a6d3a2e542c000

class Field(object):    def __init__(self, name, column_type):        self.name = name        self.column_type = column_type    def __str__(self):        return '<%s:%s>' % (self.__class__.__name__, self.name)class StringField(Field):    def __init__(self, name):        super(StringField, self).__init__(name, 'varchar(100)')class IntegerField(Field):    def __init__(self, name):        super(IntegerField, self).__init__(name, 'bigint')class ModelMetaclass(type):    def __new__(cls, name, bases, attrs):        if name=='Model':            return type.__new__(cls, name, bases, attrs)#cls代表类本身，name.bases以及attrs分别是类的名字以及，基类，和属性        print('Found model: %s' % name)        mappings = dict()        print(cls)        print(name)        print(bases)        print(attrs)        for k, v in attrs.items():            if isinstance(v, Field):                print('Found mapping: %s ==> %s' % (k, v))                mappings[k] = v        for k in mappings.keys():            attrs.pop(k) #删除改键值对        attrs['__mappings__'] = mappings # 保存属性和列的映射关系        attrs['__table__'] = name # 假设表名和类名一致        return type.__new__(cls, name, bases, attrs)  #返回修改后的类class Model(dict, metaclass=ModelMetaclass):    def __init__(self, **kw):        super(Model, self).__init__(**kw) #实例生成时使用元类修改类的属性    def __getattr__(self, key):        try:            return self[key]        except KeyError:            raise AttributeError(r"'Model' object has no attribute '%s'" % key)        def __setattr__(self, key, value):  #给属性赋值时会调用该方法        self[key] = value    def save(self):        fields = []        params = []        args = []        for k, v in self.__mappings__.items():            fields.append(v.name)            params.append('?')            args.append(getattr(self, k, None))        sql = 'insert into %s (%s) values (%s)' % (self.__table__, ','.join(fields), ','.join(params))        print('SQL: %s' % sql)        print('ARGS: %s' % str(args))# testing code:class User(Model):    id = IntegerField('id')    name = StringField('username')    email = StringField('email')    password = StringField('password')u = User(id=12345, name='Michael', email='test@orm.org', password='my-pwd')u.save()

OUTPUT:

Found model: User<class '__main__.ModelMetaclass'> #cls的值User (<class '__main__.Model'>,) #User的父类{'__module__': '__main__', '__qualname__': 'User', 'id': <__main__.IntegerField object at 0x000002186BD17550>, 'name': <__main__.StringField object at 0x000002186BD17F28>, 'email': <__main__.StringField object at 0x000002186BE893C8>, 'password': <__main__.StringField object at 0x000002186BE89BA8>}Found mapping: id ==> <IntegerField:id>  #User 的 attrsFound mapping: name ==> <StringField:username>Found mapping: email ==> <StringField:email>Found mapping: password ==> <StringField:password>SQL: insert into User (id,username,email,password) values (?,?,?,?)ARGS: [12345, 'Michael', 'test@orm.org', 'my-pwd']

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