title: Airflow callbacks for tasks
body: <p>I want to run a python function in two situations </p>
  
  <ol>
  <li>When Dag runs means when someone triggers the dag</li>
  <li>When every task starts</li>
  </ol>
  
  <p>I know airflow provides the on_success_callback and on_failure_callback
  but these are when task completes or fails.</p>