title: Runtime configuration to PythonOperator
body: <p>I can't find any working documentation on how to use the JSON variables passed with "-c" for e.g. a backfill job. </p>
  
  <p>I've been printing my python tasks **kwargs to find out, but I still can't determine it. provide_context=True</p>
  
  <p>Can anyone point me in the right direction?</p>
  
  <p>So, what I want to do:</p>
  
  <pre><code>airflow backfill mydag -c '{"override":"yes"}' -s 2018-12-01 -e 2018-12-12
  </code></pre>
  
  <p>I have a PythonOperator:</p>
  
  <pre><code>PythonOperator(
      task_id = 'task_identifier',
      python_callable = 'run_task',
      op_kwargs = {
          'task_conf': task_config 
      },
      provide_context=True,
      dag = this_dag
  )
  </code></pre>
  
  <p>Within run_task, I would like to access the override variable:</p>
  
  <pre><code>def run_task(*args, **kwargs): 
  
      dag_run = kwargs.get('dag_run')
      logging.info(kwargs['dag_run'].conf.get('override'))
  </code></pre>
  
  <p>But I can't find a method to access this override variable</p>
  
  <pre><code>[2018-12-17 10:07:24,649] {models.py:1760} ERROR - 'NoneType' object has no attribute 'get'
  Traceback (most recent call last):
    File "/home/daimonie/.local/lib/python3.6/site-packages/airflow/models.py", line 1659, in _run_raw_task
      result = task_copy.execute(context=context)
    File "/home/daimonie/.local/lib/python3.6/site-packages/airflow/operators/python_operator.py", line 95, in execute
      return_value = self.execute_callable()
    File "/home/daimonie/.local/lib/python3.6/site-packages/airflow/operators/python_operator.py", line 100, in execute_callable
      return self.python_callable(*self.op_args, **self.op_kwargs)
    File "/home/daimonie/airflow/dags/dag_scheduled_queries.py", line 65, in run_query
      logging.info(kwargs['dag_run'].conf.get('override'))
  </code></pre>
  
  <p>Edit: I did find a config setting and description that seems to indicate that these parameters need to already been set in the code</p>
  
  <pre><code># Whether to override params with dag_run.conf. If you pass some key-value pairs through `airflow backfill -c` or
  # `airflow trigger_dag -c`, the key-value pairs will override the existing ones in params.
  dag_run_conf_overrides_params=True
  </code></pre>
  
  <p>The donot_pickle parameter was set to False.</p>