[Python] ImportError : cannot import DAG airflow

I have simple code, I am trying to import DAG from airflow

from airflow import DAG
from airflow.operators import BashOperator,S3KeySensor
from datetime import datetime, timedelta
import psycopg2
from datetime import date, timedelta

yesterday = date.today() - timedelta(1)
yesterdayDate = yesterday.strftime('%Y-%m-%d')


But, I am getting Import Error

Traceback (most recent call last):
File "airflow.py", line 9, in <module>
from airflow import DAG
File "/home/ubuntu/airflow/dags/airflow.py", line 9, in <module>
from airflow import DAG
ImportError: cannot import name DAG


apache-airflow version installed 1.9.0

I don't know where I am going wrong

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