[AIRFLOW-5798] Set default to None for external_task_id in ExternalTaskSensor

[vladimir-gavrilenko]

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Description

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Pycharm highlighted my code when I used ExternalTaskSensor to wait for an entire DAG completion.

Two options here: use str or None or Optional[str]. I can switch to the second option if necessary.

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No tests for docs

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[codecov-io]

Codecov Report

Merging #6431 into master will decrease coverage by 0.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6431      +/-   ##
==========================================
- Coverage   84.26%   83.39%   -0.87%     
==========================================
  Files         627      627              
  Lines       36506    36549      +43     
==========================================
- Hits        30760    30481     -279     
- Misses       5746     6068     +322

Impacted Files	Coverage Δ
airflow/sensors/external_task_sensor.py	85.71% <ø> (ø)	⬆️
airflow/operators/postgres_operator.py	0% <0%> (-100%)	⬇️
...ow/contrib/example_dags/example_qubole_operator.py	32.5% <0%> (-57.5%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/executors/sequential_executor.py	47.61% <0%> (-52.39%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
airflow/example_dags/example_python_operator.py	63.33% <0%> (-31.12%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
... and 32 more

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[mik-laj]

Looking at the constructor definitions, this parameter is required. Why do you think it's optional? Should we not also change the function definitions?
https://github.com/apache/airflow/blob/d719e1f/airflow/sensors/external_task_sensor.py#L65

[vladimir-gavrilenko]

@mik-laj , this parameter is required, but if you want to wait for the entire DAG completion, not only the specific task, you should explicitly set this parameter to None. Pls see this: https://github.com/apache/airflow/blob/d719e1f/airflow/sensors/external_task_sensor.py#L38-L39

But should we also change the function definition is a good question, actually :-)

[feluelle]

But should we also change the function definition is a good question, actually :-)

That's really a (somehow) tough question.
I would personally say
yes, because using `None`` as default is quite common.
no, because actually we want the user to define a task_id according to the class name.
yes. :D

[feluelle]

@vladimir-gavrilenko I created a Jira ticket because it contains changes to code.