Parse xcom in local function in airflow

Question

I'm running an Airflow DAG with tasks executed via DockerOperator. I need to fetch the result from first task in the second. This can be done with xcoms in a following way:

1. set first tasks 'xcom_all=True'

2. print output from first task

3. fetch xcom in the environment field of second task, something like:

   environment={'xcom_from_previous':'{{ task_instance.xcom_pull("first_task") }}'}

4. Get the value in second task via os.environ.

Now, all is fine but my xcom output seems to be too large, so that what I read in the second task gets truncated, and is not useable, of course.

So, I could parse the '{{ task_instance.xcom_pull("first_task") }}' part before creating the environment, but how do I create a local function that actually can parse the template? Because, the field 'environment' in dockeroperator is templated, but regular functions in the DAG are not?

Currently, unfortunately forced to use airflow v 2.2.5 w

Hope my problem is clear.

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Update:

If one needs to parse xcom or any template data, use user_defined_filters. But in current case, it was not actually needed. The problem was in simple de-serialization of string and looking at data in the airflow log.