title: Dynamic tasks in airflow based on an external file
body: <p>I am reading list of elements from an external file and looping over elements to create a series of tasks.</p>
  <p>For example, if there are 2 elements in the file - [A, B]. There will be 2 series of tasks:</p>
  <pre><code>A1 -&gt; A2 ..
  B1 -&gt; B2 ...
  </code></pre>
  <p>This reading elements logic is not part of any task but in the DAG itself. Thus Scheduler is calling it many times a day while reading the DAG file. I want to call it only during DAG runtime.</p>
  <p>Wondering if there is already a pattern for such kind of use cases?</p>