title: Airflow: Get previous task id in the next task
body: <p>I have 2 tasks. In the first, python operator computes something and in the second I want to use the output of the python operator in the Http operator. Here is my code:</p>
  
  <pre><code>source_list = ['account', 'sales']
  
  for source_type in source_list:
      t2 = PythonOperator(
                  task_id='compute_next_gather_time_for_' + source_type,
                  python_callable=compute_next_gather_time,
                  provide_context=True,
                  trigger_rule=TriggerRule.ALL_SUCCESS,
                  op_args=[source_type],
                  retries=3
              )
  
      t3 = SimpleHttpOperator(
                  task_id='request_' + source_type + '_report',
                  method='POST',
                  http_conn_id='abc',
                  endpoint=endpoint,
                  data=json.dumps({
                      "query": {
                          "start": "{{ task_instance.xcom_pull(task_ids='prev_task_id') }}",
                          "stop": str(yesterday),
                          "fields": [
                              1
                          ]
                      }
                  }),
                  headers={"Content-Type": "application/json", "Authorization": 'abc'},
                  response_check=lambda response: True if len(response.json()) == 0 else False,
                  log_response=True,
                  retries=3
              )
  </code></pre>
  
  <p>Query: I want to pass previous task id in t3 in its data variable. I am not sure how to do that since t2 task id is not constant. It changes with changing source_type. Evidently, when I tried it did not render it.</p>