airflow/operators/branch.py

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"""Branching operators."""

from __future__ import annotations

from typing import TYPE_CHECKING, Iterable

from airflow.models.baseoperator import BaseOperator
from airflow.models.skipmixin import SkipMixin

if TYPE_CHECKING:
    from airflow.models import TaskInstance
    from airflow.serialization.pydantic.taskinstance import TaskInstancePydantic
    from airflow.utils.context import Context


class BranchMixIn(SkipMixin):
    """Utility helper which handles the branching as one-liner."""

    def do_branch(self, context: Context, branches_to_execute: str | Iterable[str]) -> str | Iterable[str]:
        """Implement the handling of branching including logging."""
        self.log.info("Branch into %s", branches_to_execute)
        branch_task_ids = self._expand_task_group_roots(context["ti"], branches_to_execute)
        self.skip_all_except(context["ti"], branch_task_ids)
        return branches_to_execute

    def _expand_task_group_roots(
        self, ti: TaskInstance | TaskInstancePydantic, branches_to_execute: str | Iterable[str]
    ) -> Iterable[str]:
        """Expand any task group into its root task ids."""
        if TYPE_CHECKING:
            assert ti.task

        task = ti.task
        dag = task.dag
        if TYPE_CHECKING:
            assert dag

        if branches_to_execute is None:
            return
        elif isinstance(branches_to_execute, str) or not isinstance(branches_to_execute, Iterable):
            branches_to_execute = [branches_to_execute]

        for branch in branches_to_execute:
            if branch in dag.task_group_dict:
                tg = dag.task_group_dict[branch]
                root_ids = [root.task_id for root in tg.roots]
                self.log.info("Expanding task group %s into %s", tg.group_id, root_ids)
                yield from root_ids
            else:
                yield branch


class BaseBranchOperator(BaseOperator, BranchMixIn):
    """
    A base class for creating operators with branching functionality, like to BranchPythonOperator.

    Users should create a subclass from this operator and implement the function
    `choose_branch(self, context)`. This should run whatever business logic
    is needed to determine the branch, and return one of the following:
    - A single task_id (as a str)
    - A single task_group_id (as a str)
    - A list containing a combination of task_ids and task_group_ids

    The operator will continue with the returned task_id(s) and/or task_group_id(s), and all other
    tasks directly downstream of this operator will be skipped.
    """

    def choose_branch(self, context: Context) -> str | Iterable[str]:
        """
        Abstract method to choose which branch to run.

        Subclasses should implement this, running whatever logic is
        necessary to choose a branch and returning a task_id or list of
        task_ids.

        :param context: Context dictionary as passed to execute()
        """
        raise NotImplementedError

    def execute(self, context: Context):
        return self.do_branch(context, self.choose_branch(context))