feat: support key: +{task}[{xcom_key}] use

[yanshil]

Airflow supports use of {decorated_task}[{xcom_key}] for xcoms that are not return_value link

Here I support for fetching upstream task's xcom from decorated task with multiple_outputs=True

The following example has already been tested in Airflow 3.1.0 (and should also works in Airflow 3)

example_multiple_outputs_taskflow:
  schedule: null
  render_template_as_native_obj: True
  catchup: false
  tags: [example, test]
  tasks:
    - task_id: collect
      multiple_outputs: true
      decorator: airflow.decorators.task
      python_callable: sample.collect
    - task_id: echo1
      decorator: airflow.decorators.task
      python_callable: sample.echo
      value: "+collect['key1']"
    - task_id: echo2
      decorator: airflow.decorators.task
      python_callable: sample.echo
      value: "+collect['key2']"

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.65%. Comparing base (294bc47) to head (6f8177a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dagfactory/dagbuilder.py	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #593      +/-   ##
==========================================
- Coverage   93.77%   93.65%   -0.13%     
==========================================
  Files          13       13              
  Lines        1125     1135      +10     
==========================================
+ Hits         1055     1063       +8     
- Misses         70       72       +2     

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[pankajastro]

Could you please add an example DAG here and document this feature?

[Copilot]

Pull Request Overview

This PR adds support for accessing XCom values from decorated tasks with multiple_outputs=True using the syntax +{task}[{xcom_key}]. This extends the existing +{task} functionality to allow fetching specific keys from tasks that return multiple outputs.

• Refactored XCom value replacement logic into a dedicated helper method
• Added regex pattern matching to support the new +{task}[{xcom_key}] syntax
• Updated existing functionality to use the new consolidated approach

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
dev/dags/sample.py	Added sample functions to demonstrate the new XCom key syntax
dagfactory/dagbuilder.py	Implemented regex-based parsing for XCom references and consolidated replacement logic

---

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[Copilot]

The method signature uses dict(str, Any) which is incorrect syntax. Should be Dict[str, Any] and requires importing Dict from typing.

[Copilot]

The regex pattern allows empty task names with \w*. This should be \w+ to require at least one word character for the task name, preventing matches on invalid references like +[] or +['key'].

Suggested change

	_PATTERN = re.compile(r"^\+(?P<task>\w*)(\[['\"](?P<xcom_key>.*)['\"]\])?$")
	_PATTERN = re.compile(r"^\+(?P<task>\w+)(\[['\"](?P<xcom_key>.*)['\"]\])?$")

[Copilot]

The regex pattern (?P<xcom_key>.*) allows any characters including newlines and control characters in the xcom_key. Consider using [^'\"]* or similar to restrict allowed characters and prevent potential injection issues.

Suggested change

	_PATTERN = re.compile(r"^\+(?P<task>\w*)(\[['\"](?P<xcom_key>.*)['\"]\])?$")
	_PATTERN = re.compile(r"^\+(?P<task>\w*)(\[['\"](?P<xcom_key>[^'\"]*)['\"]\])?$")

[yanshil]

I also add a simple feature docs. You can reorder it if you want.