title: Airflow: PythonOperator: why to include 'ds' arg?
body: <p>While defining a function to be later used as a python_callable, why is 'ds' included as the first arg of the function?</p>
  
  <p>For example:</p>
  
  <pre><code>def python_func(ds, **kwargs):
      pass
  </code></pre>
  
  <p>I looked into the Airflow documentation, but could not find any explanation.</p>