title: ImportError : cannot import DAG airflow
body: <p>I have simple code, I am trying to import DAG from airflow</p>
  
  <pre><code>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')
  </code></pre>
  
  <p>But, I am getting Import Error</p>
  
  <pre><code>Traceback (most recent call last):
    File "airflow.py", line 9, in &lt;module&gt;
      from airflow import DAG
    File "/home/ubuntu/airflow/dags/airflow.py", line 9, in &lt;module&gt;
      from airflow import DAG
  ImportError: cannot import name DAG
  </code></pre>
  
  <p>apache-airflow version installed 1.9.0</p>
  
  <p>I don't know where I am going wrong</p>