I got value type 'sqlalchemy.orm.attributes.InstrumentedAttribute'

I'm using sqlalchemy to my Flask application and run in python 3.7. Basically I just want only to get the value of self.tag_str and assume that the value of this variable is would be str but instead I got the value of type <class 'sqlalchemy.orm.attributes.InstrumentedAttribute'> which is not valid to convert in to json since json.loads accepts only string value. So I want to know if there is a way that i can get the original value ?

class TestModel(InternalJsonSerializer, CommonIntegraColumns):
    __tablename__ = 'test_table_name'
    __type__ = 'test_type'

    id = Column('test1', BigInteger, primary_key=True, autoincrement=True)
    col1 = Column('test2', String(2), nullable=False)
    col2 = Column(
        'test3', String(100), nullable=False)
    col3 = Column('test4', String(100), nullable=False)
    tag_str = Column('test5', Text, nullable=True)

    @hybrid_property
    def environ(self):

        # @tag_str InstrumentedAttribute

        if self.tag_str:
            val = json.loads(str(self.tag_str))
            return val

        return None

    @environ.setter
    def set_environ(self, value):
        if value is None:
            self.tag_str = None
        else:
            self.tag_str = json.dumps(value)