元類實例，操作數據庫

class ModelMetaclass(type):

    def __new__(cls, name, bases, attrs):
        mappings = dict()
        # 判斷是否需要保存
        for k, v in attrs.items():
            # 判斷是否是指定StringField或者IntegerField的實例對象
            if isinstance(v, tuple):
                # print('Found mapping: %s => %s' % (k, v))
                mappings[k] = v

        # 刪除這些已經在字典中存儲的屬性
        for k in mappings.keys():
            attrs.pop(k)

        attrs['__mappings__'] = mappings
        attrs['__table__'] = name
        return type.__new__(cls, name, bases, attrs)

class User(metaclass=ModelMetaclass):
    uid = ('uid', "int unsigned")
    name = ('username', "varchar(40)")
    email = ('email', "varchar(80)")
    # 當指定元類之後，以上的類屬性將不在類中，而是在__mappings__屬性指定的字典中
    # __mappings__ = {
    #     "uid" : ('uid', "int unsigned"),
    #     "name": ('username', "varchar(40)"),
    #     "email": ('email', "varchar(80)"),
    # }
    # __table__ = "User"

    def __init__(self, **kwargs):
        for name, value in kwargs.items():
            setattr(self, name, value)

    def save(self):
        fields = []
        args = []
        for k, v in self.__mappings__.items():
            fields.append(v[0])
            args.append(getattr(self, k, None))

        # sql = "insert into %s (%s) value (%s)" % (self.__table__, ','.join(fields), ','.join([str(i) for i in args]))
        args_temp = list()
        for temp in args:
            # 判斷是否是數字類型
            if isinstance(temp, int):
                args_temp.append(str(temp))
            elif isinstance(temp, str):
                args_temp.append("""'%s'""" % temp)
        sql = 'insert into %s (%s) value (%s)' % (self.__table__, ','.join(fields), ','.join(args_temp))
        print(sql)

u = User(uid=123, name="test", email="[email protected]")
u.save()