Airflow: PythonOperator: why to include 'ds' arg? : <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>
