title: Airflow tasks iterating over list should run sequentially
body: <p>I am running a tasks based on a list. The task id gets incremented based on the list. After the completion of these tasks I want another tasks to execute . Following is the code:</p>
  <pre><code>with DAG('test',) as dag:
  
          t1 = [PythonOperator(
          task_id=f&quot;task_hour_{hours}&quot;,
          python_callable=hourly_job,
          op_kwargs={
          &quot;hour&quot;: hours
          }
          ) for hours in ['01', '02', '03']
          ]
      
          t2 = PythonOperator(
          task_id=&quot;daily&quot;,
          python_callable=daily_job
          )
      t1 &gt;&gt; t2
  </code></pre>
  <p>What is happening is these <code>hourly</code> tasks are all running in parallel followed up by <code>daily</code> task for each one of them. Like this:</p>
  <pre><code>task_hour_01 &gt;&gt; daily
  task_hour_02 &gt;&gt; daily
  task_hour_03 &gt;&gt; daily
  </code></pre>
  <p>What i want to happen is that these <code>hourly</code> tasks should execute sequentially and lastly the <code>daily</code> task should execute:</p>
  <pre><code>task_hour_01 &gt;&gt; task_hour_02 &gt;&gt; task_hour_03 &gt;&gt; daily
  </code></pre>
  <p>So there are two problems:</p>
  <ul>
  <li>The tasks should run in sequence.</li>
  <li>The daily task should be the last task to execute and should run only
  once.</li>
  </ul>