:py:mod:`airflow.models.xcom_arg`
=================================

.. py:module:: airflow.models.xcom_arg


Module Contents
---------------

Classes
~~~~~~~

.. autoapisummary::

   airflow.models.xcom_arg.XComArg


.. py:class:: XComArg(operator, key = XCOM_RETURN_KEY)

   Bases: :py:obj:`airflow.models.taskmixin.DependencyMixin`

   Class that represents a XCom push from a previous operator.
   Defaults to "return_value" as only key.

   Current implementation supports
       xcomarg >> op
       xcomarg << op
       op >> xcomarg   (by BaseOperator code)
       op << xcomarg   (by BaseOperator code)

   **Example**: The moment you get a result from any operator (decorated or regular) you can ::

       any_op = AnyOperator()
       xcomarg = XComArg(any_op)
       # or equivalently
       xcomarg = any_op.output
       my_op = MyOperator()
       my_op >> xcomarg

   This object can be used in legacy Operators via Jinja.

   **Example**: You can make this result to be part of any generated string ::

       any_op = AnyOperator()
       xcomarg = any_op.output
       op1 = MyOperator(my_text_message=f"the value is {xcomarg}")
       op2 = MyOperator(my_text_message=f"the value is {xcomarg['topic']}")

   :param operator: operator to which the XComArg belongs to
   :param key: key value which is used for xcom_pull (key in the XCom table)

   .. py:method:: __eq__(self, other)

      Return self==value.


   .. py:method:: __getitem__(self, item)

      Implements xcomresult['some_result_key']


   .. py:method:: __iter__(self)

      Override iterable protocol to raise error explicitly.

      The default ``__iter__`` implementation in Python calls ``__getitem__``
      with 0, 1, 2, etc. until it hits an ``IndexError``. This does not work
      well with our custom ``__getitem__`` implementation, and results in poor
      DAG-writing experience since a misplaced ``*`` expansion would create an
      infinite loop consuming the entire DAG parser.

      This override catches the error eagerly, so an incorrectly implemented
      DAG fails fast and avoids wasting resources on nonsensical iterating.


   .. py:method:: __str__(self)

      Backward compatibility for old-style jinja used in Airflow Operators

      **Example**: to use XComArg at BashOperator::

          BashOperator(cmd=f"... { xcomarg } ...")

      :return:


   .. py:method:: roots(self)
      :property:

      Required by TaskMixin


   .. py:method:: leaves(self)
      :property:

      Required by TaskMixin


   .. py:method:: set_upstream(self, task_or_task_list, edge_modifier = None)

      Proxy to underlying operator set_upstream method. Required by TaskMixin.


   .. py:method:: set_downstream(self, task_or_task_list, edge_modifier = None)

      Proxy to underlying operator set_downstream method. Required by TaskMixin.


   .. py:method:: resolve(self, context, session = NEW_SESSION)

      Pull XCom value for the existing arg. This method is run during ``op.execute()``
      in respectable context.


   .. py:method:: iter_xcom_args(arg)
      :staticmethod:

      Return XComArg instances in an arbitrary value.

      This recursively traverse ``arg`` and look for XComArg instances in any
      collection objects, and instances with ``template_fields`` set.


   .. py:method:: apply_upstream_relationship(op, arg)
      :staticmethod:

      Set dependency for XComArgs.

      This looks for XComArg objects in ``arg`` "deeply" (looking inside
      collections objects and classes decorated with ``template_fields``), and
      sets the relationship to ``op`` on any found.