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from typing import TYPE_CHECKING, Any, Iterator, List, Optional, Sequence, Union
from airflow.exceptions import AirflowException
from airflow.models.abstractoperator import AbstractOperator
from airflow.models.taskmixin import DAGNode, DependencyMixin
from airflow.models.xcom import XCOM_RETURN_KEY
from airflow.utils.context import Context
from airflow.utils.edgemodifier import EdgeModifier
from airflow.utils.session import NEW_SESSION, provide_session
from airflow.utils.types import NOTSET
if TYPE_CHECKING:
from sqlalchemy.orm import Session
from airflow.models.operator import Operator
[docs]class XComArg(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)
"""
def __init__(self, operator: "Operator", key: str = XCOM_RETURN_KEY):
self.operator = operator
self.key = key
[docs] def __eq__(self, other):
return self.operator == other.operator and self.key == other.key
[docs] def __getitem__(self, item: str) -> "XComArg":
"""Implements xcomresult['some_result_key']"""
if not isinstance(item, str):
raise ValueError(f"XComArg only supports str lookup, received {type(item).__name__}")
return XComArg(operator=self.operator, key=item)
[docs] def __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.
"""
raise TypeError(f"{self.__class__.__name__!r} object is not iterable")
[docs] def __str__(self):
"""
Backward compatibility for old-style jinja used in Airflow Operators
**Example**: to use XComArg at BashOperator::
BashOperator(cmd=f"... { xcomarg } ...")
:return:
"""
xcom_pull_kwargs = [
f"task_ids='{self.operator.task_id}'",
f"dag_id='{self.operator.dag.dag_id}'",
]
if self.key is not None:
xcom_pull_kwargs.append(f"key='{self.key}'")
xcom_pull_kwargs = ", ".join(xcom_pull_kwargs)
# {{{{ are required for escape {{ in f-string
xcom_pull = f"{{{{ task_instance.xcom_pull({xcom_pull_kwargs}) }}}}"
return xcom_pull
@property
[docs] def roots(self) -> List[DAGNode]:
"""Required by TaskMixin"""
return [self.operator]
@property
[docs] def leaves(self) -> List[DAGNode]:
"""Required by TaskMixin"""
return [self.operator]
[docs] def set_upstream(
self,
task_or_task_list: Union[DependencyMixin, Sequence[DependencyMixin]],
edge_modifier: Optional[EdgeModifier] = None,
):
"""Proxy to underlying operator set_upstream method. Required by TaskMixin."""
self.operator.set_upstream(task_or_task_list, edge_modifier)
[docs] def set_downstream(
self,
task_or_task_list: Union[DependencyMixin, Sequence[DependencyMixin]],
edge_modifier: Optional[EdgeModifier] = None,
):
"""Proxy to underlying operator set_downstream method. Required by TaskMixin."""
self.operator.set_downstream(task_or_task_list, edge_modifier)
@provide_session
[docs] def resolve(self, context: Context, session: "Session" = NEW_SESSION) -> Any:
"""
Pull XCom value for the existing arg. This method is run during ``op.execute()``
in respectable context.
"""
result = context["ti"].xcom_pull(
task_ids=self.operator.task_id, key=str(self.key), default=NOTSET, session=session
)
if result is NOTSET:
raise AirflowException(
f'XComArg result from {self.operator.task_id} at {context["ti"].dag_id} '
f'with key="{self.key}" is not found!'
)
return result
@staticmethod
[docs] def iter_xcom_args(arg: Any) -> Iterator["XComArg"]:
"""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.
"""
if isinstance(arg, XComArg):
yield arg
elif isinstance(arg, (tuple, set, list)):
for elem in arg:
yield from XComArg.iter_xcom_args(elem)
elif isinstance(arg, dict):
for elem in arg.values():
yield from XComArg.iter_xcom_args(elem)
elif isinstance(arg, AbstractOperator):
for elem in arg.template_fields:
yield from XComArg.iter_xcom_args(elem)
@staticmethod
[docs] def apply_upstream_relationship(op: "Operator", arg: Any):
"""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.
"""
for ref in XComArg.iter_xcom_args(arg):
op.set_upstream(ref.operator)
