PythonOperator does not work on helm installation environment

[sakaia]

Following sample code with BashOperator works but PythonOperator does not works.
When I call python_task (i.e, remove #), It fails as follows.
I am ruuning on airflow 1.10.12 with helm installation.

from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator
from airflow.operators.python_operator import PythonOperator
from airflow.operators.bash_operator import BashOperator

from datetime import datetime

def my_func():
    print('Hello from my_func')

with DAG('python_dag', description='Python DAG',start_date=datetime(2018, 11, 1), catchup=False) as dag:
        dummy_task      = DummyOperator(task_id='dummy_task', retries=3)
        ## following operator works fine
        bash_task       = BashOperator( task_id='print_date', bash_command='date', dag=dag)
        dummy_task >> bash_task
        ## following operator does not work
        #python_task     = PythonOperator(task_id='python_task', python_callable=my_func)
        #dummy_task >> python_task

Traceback (most recent call last):
  File "/home/airflow/.local/bin/airflow", line 37, in <module>
    args.func(args)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/utils/cli.py", line 81, in wrapper
    return f(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.6/site-packages/airflow/bin/cli.py", line 618, in run
    dag_pickle = session.query(DagPickle).filter(DagPickle.id == args.pickle).first()
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 3429, in first
    ret = list(self[0:1])
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 3203, in __getitem__
    return list(res)
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 100, in instances
    cursor.close()
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 80, in instances
    rows = [proc(row) for row in fetch]
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 80, in <listcomp>
    rows = [proc(row) for row in fetch]
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 588, in _instance
    populators,
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/orm/loading.py", line 725, in _populate_full
    dict_[key] = getter(row)
  File "/home/airflow/.local/lib/python3.6/site-packages/sqlalchemy/sql/sqltypes.py", line 1723, in process
    return loads(value)
  File "/home/airflow/.local/lib/python3.6/site-packages/dill/_dill.py", line 283, in loads
    return load(file, ignore, **kwds)
  File "/home/airflow/.local/lib/python3.6/site-packages/dill/_dill.py", line 278, in load
    return Unpickler(file, ignore=ignore, **kwds).load()
  File "/home/airflow/.local/lib/python3.6/site-packages/dill/_dill.py", line 481, in load
    obj = StockUnpickler.load(self)
  File "/home/airflow/.local/lib/python3.6/site-packages/dill/_dill.py", line 471, in find_class
    return StockUnpickler.find_class(self, module, name)
ModuleNotFoundError: No module named 'unusual_prefix_a05a97b4444c348d2eecd28622afa81f72b1f848_pt'