title: AirFlowException - Python_Callable must be callable
body: <p>I made a small change to an existing workflow, and it has broken airflow. Here is the code:</p>
  
  <pre><code>dag_name = platform + "_" + report['table']
  
  dag = DAG(
      dag_name,
      catchup=True,
      default_args=default_args,
      schedule_interval=report['schedule']
  )
  
  with dag:
  
      trigger_report = PythonOperator(
          task_id=dag.dag_id + '_trigger_report',
          python_callable=trigger_report,
          provide_context=True,
          op_kwargs={
              'report_name': report['report'],
              'amazonmws_conn_id': default_args['amazonmws_conn_id']
          },
          dag=dag
      )
  </code></pre>
  
  <p>Here is the error I'm receiving:</p>
  
  <p><code>airflow.exceptions.AirflowException: python_callable param must be callable</code></p>