Get an XCom value in the Airflow on_failure_callback function

When we want to get an XCom variable in the on_failure_callback, we will face a nasty bug. Using the ti key to retrieve a value from the context gives us a None value.

# This does not work

It happens because in the on_failure_callback the task instance is passed to the function using the task_instance key. Therefore to get a value from XCom, we must execute this code:

task: TaskInstance = context.get('task_instance')

Alternatively, we can import XCom and access it directly:

from airflow.models import XCom
XCom.get_one(execution_date = context.get('execution_date'), key='test')
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